fix(#808): never borrow a session identity for non-tmux senders

When MOSAIC_AGENT_NAME is unset and the sender has no tmux client context
($TMUX empty), agent-send.sh fell back to 'tmux display-message -p #S',
which outside tmux reports the LAST-ACTIVE session on the server. A
nameless sender was silently stamped as whichever seat touched tmux most
recently, forging that seat's identity in the preamble.

Incident (2026-08-20, host sb-it-1-dt): a nameless non-tmux sender was
stamped 'peggy', a live seat, on a message delivered TO peggy. Reproduced
end-to-end before the fix; capture in the seat evidence note.

Fix: the display-message fallback now runs only when $TMUX is set (inside
tmux it resolves the caller's own session — the case #808 built it for).
Outside tmux with no name, the label is host:unverified. Deliberate
identity remains available via -S. Chosen over refusing the send: nameless
senders are legitimate; the defect was borrowing an identity, not lacking
one.

Tests: suite made hermetic (TMUX controlled explicitly; results no longer
depend on the caller's context — previously tests 9/10 inherited the
caller's TMUX). New case 9b: no-tmux sender is labeled unverified and
never a borrowed name (red on original: host:local-agent borrowed from the
fake tmux server; green after). 19/19 in both caller contexts.

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

@@ -126,7 +126,16 @@ if [ -z "$SRC_LABEL" ]; then
   src_host=$(hostname -s 2>/dev/null || echo "?")
   src_sess=${MOSAIC_AGENT_NAME:-}
   if [ -z "$src_sess" ]; then
-    src_sess=$(tmux display-message -p '#S' 2>/dev/null || echo "?")
+    if [ -n "${TMUX:-}" ]; then
+      # Inside tmux: display-message resolves against this client's own session.
+      src_sess=$(tmux display-message -p '#S' 2>/dev/null || echo "?")
+    else
+      # Outside tmux with no name: display-message reports the LAST-ACTIVE
+      # session — someone else's identity (measured 2026-08-20: a nameless
+      # non-tmux sender was stamped "peggy", a live seat, forged silently).
+      # Stamp an explicit unverified label instead; deliberate senders use -S.
+      src_sess="unverified"
+    fi
   fi
   SRC_LABEL="${src_host}:${src_sess}"
 fi