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/session-deleted-handler.ts

@@ -0,0 +1,50 @@
+import type { TmuxConfig } from "../../config/schema"
+import type { TrackedSession } from "./types"
+import { log } from "../../shared"
+import { queryWindowState } from "./pane-state-querier"
+import { decideCloseAction, type SessionMapping } from "./decision-engine"
+import { executeAction } from "./action-executor"
+
+export interface SessionDeletedHandlerDeps {
+  tmuxConfig: TmuxConfig
+  serverUrl: string
+  sourcePaneId: string | undefined
+  sessions: Map<string, TrackedSession>
+  isEnabled: () => boolean
+  getSessionMappings: () => SessionMapping[]
+  stopPolling: () => void
+}
+
+export async function handleSessionDeleted(
+  deps: SessionDeletedHandlerDeps,
+  event: { sessionID: string },
+): Promise<void> {
+  if (!deps.isEnabled()) return
+  if (!deps.sourcePaneId) return
+
+  const tracked = deps.sessions.get(event.sessionID)
+  if (!tracked) return
+
+  log("[tmux-session-manager] onSessionDeleted", { sessionId: event.sessionID })
+
+  const state = await queryWindowState(deps.sourcePaneId)
+  if (!state) {
+    deps.sessions.delete(event.sessionID)
+    return
+  }
+
+  const closeAction = decideCloseAction(state, event.sessionID, deps.getSessionMappings())
+  if (closeAction) {
+    await executeAction(closeAction, {
+      config: deps.tmuxConfig,
+      serverUrl: deps.serverUrl,
+      windowState: state,
+    })
+  }
+
+  deps.sessions.delete(event.sessionID)
+
+  if (deps.sessions.size === 0) {
+    deps.stopPolling()
+  }
+}