Replace the terminal `exec tmux` with a plain `tmux new-session -d` so the
launcher continues running after creating the pane. The script then resolves
the pane PID via `tmux list-panes -F '#{pane_pid}'` (with a brief retry loop)
and spawns a detached, runtime-agnostic heartbeat sidecar via `setsid bash -c
... &` + `disown`. The sidecar loops while `kill -0 <pane_pid>` succeeds,
writing ~/.config/mosaic/fleet/run/<AGENT>.hb atomically (tmp + mv) every
MOSAIC_HEARTBEAT_INTERVAL seconds (default 15), then exits naturally when the
runtime process dies — making `mosaic fleet ps` show stale then dead.
HB_RUN_DIR and interval are configurable via env; sidecar startup is
best-effort (failures warn but do not abort the launch). Two new shell tests
cover pane-PID resolution (test 6, real tmux) and sidecar invocation
correctness (test 7, fake-tmux + fake-setsid shims).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RMoEx7hfdFGjUiCHuN1RRi
