feat(tmux): sweep stale omo-agents-<pid> sessions on first spawn

Follow-up to PR #3507 addressing the Oracle-noted operational limitation:
per-PID isolated session names (getIsolatedSessionName(process.pid)) mean
that when an opencode process is SIGKILL'd (or the machine hard-reboots),
the old omo-agents-<old-pid> tmux session survives forever because nothing
is around to kill it.

Added sweepStaleOmoAgentSessions() that:
1. Lists tmux sessions matching /^omo-agents-(\d+)$/
2. For each, checks process.kill(pid, 0) to detect a dead PID
3. Skips our own PID
4. Calls killTmuxSessionIfExists for every session whose owner process is gone

Wired into TmuxSessionManager.onSessionCreated() as a one-shot (guarded by
staleSweepCompleted flag) so it runs lazily on the first subagent spawn when
isolation="session". The flag is reset in cleanup() so subsequent process
restarts re-run the sweep.

6 new tests cover: outside-tmux no-op, no matching sessions, multiple dead
PIDs, current PID skip, live PID skip, list-sessions failure.

Manual E2E verified on real tmux:
- Created omo-agents-99999, sweep killed it
- Spawned our own omo-agents-<pid>, closeTmuxPane returned true even after
  pane auto-destroy from Ctrl+C
- Final tmux list-sessions shows zero omo-agents-* orphans

src/features/tmux-subagent/manager.test.ts

@@ -58,6 +58,7 @@ const mockSpawnTmuxSession = mock<(
   paneId: '%isolated-session',
 }))
 const mockKillTmuxSessionIfExists = mock<(sessionName: string) => Promise<boolean>>(async () => true)
+const mockSweepStaleOmoAgentSessions = mock<() => Promise<number>>(async () => 0)
 const mockIsInsideTmux = mock<() => boolean>(() => true)
 const mockGetCurrentPaneId = mock<() => string | undefined>(() => '%0')
 
@@ -102,6 +103,7 @@ mock.module('../../shared/tmux', () => {
     spawnTmuxSession: mockSpawnTmuxSession,
     killTmuxSessionIfExists: mockKillTmuxSessionIfExists,
     getIsolatedSessionName: (pid: number = 12345) => `omo-agents-${pid}`,
+    sweepStaleOmoAgentSessions: mockSweepStaleOmoAgentSessions,
   }
 })