fix(tmux): locate the REPL input box by shape, not by a Claude-only glyph

send-message.sh confirmed delivery by grepping the captured pane for a
prompt glyph (`❯`, a leading `>`, or `│ >`). Those are Claude Code shapes.
pi draws its input box as two horizontal `─` rules with the input between
them and no glyph anywhere, so on a pi pane the grep matched nothing, the
`delivered` and `draft` branches were unreachable, and every send exited 2
with "may be UNDELIVERED" after burning all its retries — on messages that
had in fact been delivered.

Measured on five live seats: pi seats vision, tess and medic returned 0
glyph matches on non-empty captures (3653 / 1086 / 1890 bytes); Claude
seats fred and tuesday returned 3 and 1.

This is a different arm from #1257. On the measured host
`tmux capture-pane -t <session> -p` and `-t <session>:0.0 -p` both return
the same 3653 bytes, so #1257's empty-capture mechanism does not reproduce
here; fixing only that would leave every pi send at rc=2 and would verify
green against a Claude pane.

The probe becomes locate_input_box(), which returns a status code instead
of a string. Found-but-empty is a real answer (an empty input box is what
a submitted message leaves behind) and is now distinguishable from
not-found, which the old `[ -z "$promptline" ]` test conflated. Box
detection anchors on the LAST pair of `─` rules: 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 in that one function. A missing
shape does not degrade gracefully — it turns every send to that runtime
into a false "may be UNDELIVERED", which is exactly what this fixes.

Tests: two fixtures added to test-send-message-verdict.sh driving a
glyphless box-drawn REPL, one that submits (expect exit 0 delivered) and
one that holds the text in the box (expect exit 2 draft). Patched suite
PASS=5 FAIL=0. Control against the pristine origin/next send-message.sh:
PASS=3 FAIL=2, fixture 4 failing as `rc=2 ... REPL input prompt not
locatable after 3 attempts — message may be UNDELIVERED`, which is the
defect reproduced in a test. Sibling suites unchanged and green:
test-send-message-socket.sh rc=0, agent-send.test.sh PASS=19 FAIL=0.

Note the suite is in test-enumeration-exclusions.txt (the CI image ships
no tmux), so these fixtures are verified locally only. That exclusion and
its #1017 burndown reason are unchanged by this commit.

Not addressed here: auto-submit-drafts.sh lines 20 and 26 carry the same
Claude-only `❯` assumption. Separate surface, separate change.

Closes #1362

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 ]