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/index.ts

@@ -1,4 +1,14 @@
 export * from "./manager"
+export * from "./event-handlers"
+export * from "./polling"
+export * from "./cleanup"
+export * from "./session-created-event"
+export * from "./session-created-handler"
+export * from "./session-deleted-handler"
+export * from "./polling-constants"
+export * from "./session-status-parser"
+export * from "./session-message-count"
+export * from "./session-ready-waiter"
 export * from "./types"
 export * from "./pane-state-querier"
 export * from "./decision-engine"