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/shared/tmux/tmux-utils/stale-session-sweep.ts

@@ -0,0 +1,72 @@
+const STALE_SESSION_PATTERN = /^omo-agents-(\d+)$/
+
+function isProcessAlive(pid: number): boolean {
+	try {
+		process.kill(pid, 0)
+		return true
+	} catch (error) {
+		const err = error as NodeJS.ErrnoException
+		return err?.code === "EPERM"
+	}
+}
+
+async function listOmoAgentSessions(tmux: string): Promise<string[]> {
+	const { spawn } = await import("./spawn-process")
+	const proc = spawn([tmux, "list-sessions", "-F", "#{session_name}"], {
+		stdout: "pipe",
+		stderr: "pipe",
+	})
+	const [stdout, , exitCode] = await Promise.all([
+		new Response(proc.stdout).text(),
+		new Response(proc.stderr).text(),
+		proc.exited,
+	])
+
+	if (exitCode !== 0) {
+		return []
+	}
+
+	return stdout
+		.split("\n")
+		.map((line) => line.trim())
+		.filter((name) => STALE_SESSION_PATTERN.test(name))
+}
+
+export async function sweepStaleOmoAgentSessions(): Promise<number> {
+	const [{ log }, { isInsideTmux }, { getTmuxPath }, { killTmuxSessionIfExists }] = await Promise.all([
+		import("../../logger"),
+		import("./environment"),
+		import("../../../tools/interactive-bash/tmux-path-resolver"),
+		import("./session-kill"),
+	])
+
+	if (!isInsideTmux()) {
+		return 0
+	}
+
+	const tmux = await getTmuxPath()
+	if (!tmux) {
+		return 0
+	}
+
+	const candidateSessions = await listOmoAgentSessions(tmux)
+	let killedCount = 0
+
+	for (const sessionName of candidateSessions) {
+		const pidMatch = sessionName.match(STALE_SESSION_PATTERN)
+		if (!pidMatch) continue
+
+		const pid = Number.parseInt(pidMatch[1], 10)
+		if (!Number.isFinite(pid)) continue
+		if (pid === process.pid) continue
+		if (isProcessAlive(pid)) continue
+
+		log("[sweepStaleOmoAgentSessions] killing stale session", { sessionName, deadPid: pid })
+		const killed = await killTmuxSessionIfExists(sessionName)
+		if (killed) {
+			killedCount += 1
+		}
+	}
+
+	return killedCount
+}