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-status-parser.ts

@@ -0,0 +1,17 @@
+type SessionStatus = { type: string }
+
+export function parseSessionStatusMap(data: unknown): Record<string, SessionStatus> {
+  if (typeof data !== "object" || data === null) return {}
+  const record = data as Record<string, unknown>
+
+  const result: Record<string, SessionStatus> = {}
+  for (const [sessionId, value] of Object.entries(record)) {
+    if (typeof value !== "object" || value === null) continue
+    const valueRecord = value as Record<string, unknown>
+    const type = valueRecord["type"]
+    if (typeof type !== "string") continue
+    result[sessionId] = { type }
+  }
+
+  return result
+}