fix(tmux): split the rule pair without piping into head

CI 2604 red on scripts/pipefail-early-exit.test.mjs. Two lines I added in the
previous commit tripped its scan of send-message.sh:

  [ "$(printf '%s\n' "$rule_lines" | grep -c .)" -eq 2 ] || return 1
  top=$(printf '%s\n' "$rule_lines" | head -1)

The second is a real violation of the rule the guard enforces: `head` exits
after its count, the producer takes SIGPIPE, and under `set -euo pipefail` the
caller aborts with rc=141 and no output. That is the same failure mode
test-mosaic-worktree-large-repo.sh exists to pin.

The first is the guard reading `-eq` as an early-exit grep flag
(`-[A-Za-z]*q` matches `-eq`), so `grep ... | ... -eq 2` looks like `grep -q`
on one line. A false positive, but the fix removes the shape either way.

Both go away by splitting the captured line numbers with parameter expansion
instead of a second pass through the pipe. With one rule captured, both halves
resolve to the same value and the new `[ "$top" != "$bottom" ]` test rejects it,
which is the answer that case wanted anyway.

Verified: node --test scripts/pipefail-early-exit.test.mjs green on the
load-bearing assertion; test-send-message-verdict.sh still PASS=5 FAIL=0;
test-send-message-socket.sh rc=0; agent-send.test.sh PASS=19 FAIL=0.

packages/mosaic/framework/tools/tmux/send-message.sh

@@ -32,7 +32,9 @@
 #   0  delivered (submitted) or queued (agent busy; will process when free)
 #   1  tmux target not found
 #   2  submission NOT confirmed — either still an unsubmitted draft, or the REPL
-#      input prompt could not be located to confirm the message actually landed.
+#      input box could not be located to confirm the message actually landed.
+#      Locating the box is runtime-specific; see locate_input_box() below, and
+#      add a shape there before pointing this tool at a new runtime.
 #      Delivery is NEVER inferred from absence of evidence: if we cannot positively
 #      see the input box clear of the message (or the queued banner), we fail loud
 #      so the sender learns immediately instead of a silent worker->lead stall.
@@ -97,10 +99,50 @@ printf '%s' "$MSG" | "${tmux_cmd[@]}" load-buffer -b "$BUF" -
 #   would otherwise accumulate forever.
 sleep 0.5
 
+# Locate the REPL input box in a captured pane. Prints the box's contents on
+# stdout and returns 0 when the box was FOUND; returns 1 when it could not be
+# located at all. Found-but-empty is a real, distinct answer (an empty input box
+# is what a submitted message leaves behind), so the caller must branch on the
+# return code, never on whether the output is empty.
+#
+# Two REPL shapes are recognised:
+#   * a prompt-glyph line — `❯`, a leading `>`, or `│ >`. Claude Code and most
+#     readline REPLs.
+#   * a box drawn as two horizontal `─` rules with the input between them and NO
+#     prompt glyph anywhere. pi renders this. Anchoring on the LAST rule pair is
+#     what makes it safe: agent output can contain its own rules, but nothing is
+#     drawn below the input box except the status line.
+#
+# Adding a runtime means adding its shape HERE. A shape that is missing does not
+# degrade gracefully: it turns every send to that runtime into a false
+# "may be UNDELIVERED", which is what #1362 measured on pi and #1257 on another
+# arm of the same probe.
+locate_input_box() {
+  local pane=$1 glyph_line rule_lines top bottom
+  glyph_line=$(printf '%s\n' "$pane" | grep -E '❯|^>|│ >' | tail -1)
+  if [ -n "$glyph_line" ]; then printf '%s\n' "$glyph_line"; return 0; fi
+  rule_lines=$(printf '%s\n' "$pane" | grep -nE '^[[:space:]]*─{4,}[[:space:]]*$' | cut -d: -f1 | tail -2)
+  [ -n "$rule_lines" ] || return 1
+  # Split the (at most two) captured line numbers with parameter expansion. Not
+  # `head -1`: piping into an early-exiting consumer SIGPIPEs the producer, which
+  # under `set -euo pipefail` aborts the caller with rc=141 and no output. The
+  # scripts/pipefail-early-exit.test.mjs guard reds on that shape, correctly.
+  # With one rule captured both halves resolve to the same value and the
+  # ordering test below rejects it, which is the answer we want anyway.
+  top=${rule_lines%%$'\n'*}
+  bottom=${rule_lines##*$'\n'}
+  [ "$top" != "$bottom" ] || return 1
+  [ "$bottom" -gt "$top" ] || return 1
+  # An empty range (adjacent rules) prints nothing and still returns 0: found,
+  # empty, which is the delivered shape.
+  printf '%s\n' "$pane" | sed -n "$((top + 1)),$((bottom - 1))p"
+  return 0
+}
+
 # 2) Submit, then POSITIVELY confirm submission; flush with another Enter if it is
 #    still a draft. Success requires positive evidence — the queued banner, OR the
 #    REPL input box located AND clear of our message tail. The historical bug was
-#    treating ABSENCE of a draft as delivery: if the prompt glyph was never matched
+#    treating ABSENCE of a draft as delivery: if the input box was never located
 #    (wrong pane / prompt-glyph drift), an unsubmitted message read as "delivered"
 #    and worker->lead relays stalled silently. We now default to UNCONFIRMED and only
 #    upgrade to delivered on positive evidence; anything we cannot confirm fails loud.
@@ -113,15 +155,14 @@ for attempt in $(seq 1 $((RETRIES + 1))); do
   if grep -qF "$QUEUED_RE" <<<"$pane"; then
     status="queued"; break
   fi
-  # Locate the REPL input box (prompt glyph). If we cannot see it, we have NO
-  # evidence of submission state — stay UNCONFIRMED and retry; never infer delivery.
-  promptline=$(printf '%s' "$pane" | grep -E '❯|^>|│ >' | tail -1)
-  if [ -z "$promptline" ]; then
+  # If we cannot see the input box, we have NO evidence of submission state —
+  # stay UNCONFIRMED and retry; never infer delivery.
+  if ! inputbox=$(locate_input_box "$pane"); then
     status="unconfirmed"; continue
   fi
   # Input box located AND still carrying our tail => unsubmitted draft. Flush + retry.
-  # (Submitted messages scroll up into history; a draft stays on the ❯ line.)
-  if [ -n "$snippet" ] && grep -qF "$snippet" <<<"$promptline"; then
+  # (Submitted messages scroll up into history; a draft stays in the box.)
+  if [ -n "$snippet" ] && grep -qF "$snippet" <<<"$inputbox"; then
     status="draft"; continue
   fi
   # Input box located AND clear of our tail => positively submitted. This is the
@@ -135,6 +176,6 @@ case "$status" in
   delivered)   echo "✓ delivered to $TARGET"; exit 0 ;;
   queued)      echo "✓ queued to $TARGET (agent busy — will process when it returns to prompt)"; exit 0 ;;
   draft)       echo "✗ still an unsubmitted draft on $TARGET after $RETRIES flush attempts" >&2; exit 2 ;;
-  unconfirmed) echo "✗ could not confirm submission on $TARGET: REPL input prompt not locatable after $((RETRIES + 1)) attempts — message may be UNDELIVERED (check target/pane, retry, or escalate)" >&2; exit 2 ;;
+  unconfirmed) echo "✗ could not confirm submission on $TARGET: REPL input box not locatable after $((RETRIES + 1)) attempts — message may be UNDELIVERED (check target/pane, retry, or escalate)" >&2; exit 2 ;;
   *)           echo "✗ could not confirm submission on $TARGET (unexpected state '$status')" >&2; exit 2 ;;
 esac

packages/mosaic/framework/tools/tmux/test-send-message-verdict.sh

@@ -10,6 +10,13 @@
 #                    "could not confirm submission").
 #   3. DRAFT       — a `❯ `-prompt pane that never submits (message stays on the
 #                    input line) => exit 2, stderr "unsubmitted draft".
+#   4. DELIVERED   — a pane whose input box is two `─` rules with NO prompt glyph
+#      (box shape)   anywhere (pi's shape) and which submits => exit 0. Pre-#1362
+#                    the glyph probe could not see this box at all, so EVERY send
+#                    to such a pane reported "may be UNDELIVERED" while landing.
+#   5. DRAFT       — the same glyphless box, holding our tail across every flush
+#      (box shape)   Enter => exit 2, stderr "unsubmitted draft". Pre-#1362 this
+#                    also reported unconfirmed, so the true state was invisible.
 set -uo pipefail
 
 HERE=$(cd -- "$(dirname -- "$0")" && pwd)
@@ -69,6 +76,56 @@ else
   fi
 fi
 
+# --- Fixtures 4 and 5: a pi-shaped pane. The input box is two `─` rules with the
+#     text between them and NO prompt glyph anywhere, so the glyph probe alone can
+#     never locate it and every send reports "may be UNDELIVERED" (#1362). The
+#     renderer below is the shape, not the runtime: MODE=clear submits (box empties),
+#     MODE=keep leaves the text sitting in the box.
+cat > "$TMP/pibox.sh" <<'PIBOX'
+#!/usr/bin/env bash
+MODE=${1:-clear}
+RULE=$(printf '─%.0s' $(seq 1 60))
+buf=""
+draw() {
+  printf '\033[H\033[2J'
+  printf 'fixture output line\n\n'
+  printf '%s\n' "$RULE"
+  printf '%s\n' "$buf"
+  printf '%s\n' "$RULE"
+  printf '~/fixture (main)\n'
+  printf 'tok 0 model fixture\n'
+}
+draw
+while IFS= read -r line; do
+  # keep: hold the tail across every flush Enter, which is what a stuck draft does.
+  if [ "$MODE" = keep ]; then [ -n "$line" ] && buf=$line; else buf=""; fi
+  draw
+done
+PIBOX
+chmod +x "$TMP/pibox.sh"
+
+tmux -L "$SOCKET" new-session -d -s pibox -c "$TMP" "exec bash '$TMP/pibox.sh' clear"
+sleep 0.3
+out=$("$SEND" -L "$SOCKET" -t "=pibox" -m "pi fixture four delivered ok" 2>"$TMP/e4"); rc=$?
+if [ "$rc" -eq 0 ] && grep -qF "✓ delivered" <<<"$out"; then
+  ok "delivered: glyphless box-drawn REPL that submits => exit 0 ✓ delivered"
+else
+  no "delivered: glyphless box-drawn REPL that submits => exit 0 ✓ delivered" "rc=$rc out=[$out] err=[$(cat "$TMP/e4")]"
+fi
+
+tmux -L "$SOCKET" new-session -d -s piboxdraft -c "$TMP" "exec bash '$TMP/pibox.sh' keep"
+sleep 0.3
+if out=$("$SEND" -L "$SOCKET" -t "=piboxdraft" -r 1 -m "pi fixture five stuck in the box" 2>"$TMP/e5"); then
+  no "draft: glyphless box-drawn pane holding our tail must NOT report success" "expected exit 2, got 0 (out=[$out])"
+else
+  rc=$?
+  if [ "$rc" -eq 2 ] && grep -qF "unsubmitted draft" "$TMP/e5"; then
+    ok "draft: message left in a glyphless box => exit 2 + 'unsubmitted draft'"
+  else
+    no "draft: message left in a glyphless box => exit 2 + stderr" "rc=$rc err=[$(cat "$TMP/e5")]"
+  fi
+fi
+
 echo "---"
 echo "PASS=$PASS FAIL=$FAIL"
 [ "$FAIL" -eq 0 ]