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/cleanup.ts

@@ -0,0 +1,42 @@
+import type { TmuxConfig } from "../../config/schema"
+import { log } from "../../shared"
+import type { TrackedSession } from "./types"
+import { queryWindowState } from "./pane-state-querier"
+import { executeAction } from "./action-executor"
+
+export async function cleanupTmuxSessions(params: {
+  tmuxConfig: TmuxConfig
+  serverUrl: string
+  sourcePaneId: string | undefined
+  sessions: Map<string, TrackedSession>
+  stopPolling: () => void
+}): Promise<void> {
+  params.stopPolling()
+
+  if (params.sessions.size === 0) {
+    log("[tmux-session-manager] cleanup complete")
+    return
+  }
+
+  log("[tmux-session-manager] closing all panes", { count: params.sessions.size })
+  const state = params.sourcePaneId ? await queryWindowState(params.sourcePaneId) : null
+
+  if (state) {
+    const closePromises = Array.from(params.sessions.values()).map((tracked) =>
+      executeAction(
+        { type: "close", paneId: tracked.paneId, sessionId: tracked.sessionId },
+        { config: params.tmuxConfig, serverUrl: params.serverUrl, windowState: state },
+      ).catch((error) =>
+        log("[tmux-session-manager] cleanup error for pane", {
+          paneId: tracked.paneId,
+          error: String(error),
+        }),
+      ),
+    )
+
+    await Promise.all(closePromises)
+  }
+
+  params.sessions.clear()
+  log("[tmux-session-manager] cleanup complete")
+}