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-created-event.ts

@@ -0,0 +1,44 @@
+type UnknownRecord = Record<string, unknown>
+
+function isRecord(value: unknown): value is UnknownRecord {
+  return typeof value === "object" && value !== null
+}
+
+function getNestedRecord(value: unknown, key: string): UnknownRecord | undefined {
+  if (!isRecord(value)) return undefined
+  const nested = value[key]
+  return isRecord(nested) ? nested : undefined
+}
+
+function getNestedString(value: unknown, key: string): string | undefined {
+  if (!isRecord(value)) return undefined
+  const nested = value[key]
+  return typeof nested === "string" ? nested : undefined
+}
+
+export interface SessionCreatedEvent {
+  type: string
+  properties?: { info?: { id?: string; parentID?: string; title?: string } }
+}
+
+export function coerceSessionCreatedEvent(input: {
+  type: string
+  properties?: unknown
+}): SessionCreatedEvent {
+  const properties = isRecord(input.properties) ? input.properties : undefined
+  const info = getNestedRecord(properties, "info")
+
+  return {
+    type: input.type,
+    properties:
+      info || properties
+        ? {
+            info: {
+              id: getNestedString(info, "id"),
+              parentID: getNestedString(info, "parentID"),
+              title: getNestedString(info, "title"),
+            },
+          }
+        : undefined,
+  }
+}