refactor(tmux-subagent): split manager and decision-engine into focused modules

Extract session lifecycle, polling, grid planning, and event handling:
- polling.ts: session polling controller with stability detection
- event-handlers.ts: session created/deleted handlers
- grid-planning.ts, spawn-action-decider.ts, spawn-target-finder.ts
- session-status-parser.ts, session-message-count.ts
- cleanup.ts, polling-constants.ts, tmux-grid-constants.ts

src/features/tmux-subagent/polling-constants.ts

@@ -0,0 +1,6 @@
+export const SESSION_TIMEOUT_MS = 10 * 60 * 1000
+
+// Stability detection constants (prevents premature closure - see issue #1330)
+// Mirrors the proven pattern from background-agent/manager.ts
+export const MIN_STABILITY_TIME_MS = 10 * 1000
+export const STABLE_POLLS_REQUIRED = 3