fix(fleet): watch viewer-session leak + workdir test settle-race

The 2 deferred F3 items from the #595/#599 reviews:
- agent watch: wrap the interactive attach in try/finally and kill the grouped
  viewer session in finally — so an attach that throws or an interrupted process
  never leaves a stray <agent>-watch-<pid> session behind (the leak you found).
- test-start-agent-session.sh Test 1: pane_current_path briefly reflects the tmux
  server's cwd until the pane process establishes its -c start dir; poll until it
  settles. Fixes the cwd-dependent false failure that aborted the suite before the
  heartbeat tests (6/7) — those now run.

Verified: full shell suite green from /tmp (was failing); prettier clean.

Refs #588 #542

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/mosaic/framework/tools/fleet/test-start-agent-session.sh

@@ -32,8 +32,15 @@ MOSAIC_AGENT_COMMAND='bash --noprofile --norc -i' \
   "$START" "$AGENT"
 
 tmux -L "$SOCKET" has-session -t "=$AGENT:0.0" || fail "agent session was not created"
-actual_dir=$(tmux -L "$SOCKET" display-message -p -t "=$AGENT:0.0" '#{pane_current_path}')
-[ "$actual_dir" = "$WORKDIR" ] || fail "agent workdir mismatch: $actual_dir"
+# Retry: pane_current_path briefly reflects the tmux server's cwd until the pane
+# process establishes its own cwd (the -c start dir). Poll until it settles.
+actual_dir=""
+for _ in $(seq 1 30); do
+  actual_dir=$(tmux -L "$SOCKET" display-message -p -t "=$AGENT:0.0" '#{pane_current_path}')
+  [ "$actual_dir" = "$WORKDIR" ] && break
+  sleep 0.1
+done
+[ "$actual_dir" = "$WORKDIR" ] || fail "agent workdir mismatch: $actual_dir (expected $WORKDIR)"
 
 # ── Test 2: idempotency (duplicate start prints 'already running') ─────────────
 MOSAIC_TMUX_SOCKET="$SOCKET" \