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-ready-waiter.ts

@@ -0,0 +1,44 @@
+import type { PluginInput } from "@opencode-ai/plugin"
+import {
+  SESSION_READY_POLL_INTERVAL_MS,
+  SESSION_READY_TIMEOUT_MS,
+} from "../../shared/tmux"
+import { log } from "../../shared"
+import { parseSessionStatusMap } from "./session-status-parser"
+
+type OpencodeClient = PluginInput["client"]
+
+export async function waitForSessionReady(params: {
+  client: OpencodeClient
+  sessionId: string
+}): Promise<boolean> {
+  const startTime = Date.now()
+
+  while (Date.now() - startTime < SESSION_READY_TIMEOUT_MS) {
+    try {
+      const statusResult = await params.client.session.status({ path: undefined })
+      const allStatuses = parseSessionStatusMap(statusResult.data)
+
+      if (allStatuses[params.sessionId]) {
+        log("[tmux-session-manager] session ready", {
+          sessionId: params.sessionId,
+          status: allStatuses[params.sessionId].type,
+          waitedMs: Date.now() - startTime,
+        })
+        return true
+      }
+    } catch (error) {
+      log("[tmux-session-manager] session status check error", { error: String(error) })
+    }
+
+    await new Promise<void>((resolve) => {
+      setTimeout(resolve, SESSION_READY_POLL_INTERVAL_MS)
+    })
+  }
+
+  log("[tmux-session-manager] session ready timeout", {
+    sessionId: params.sessionId,
+    timeoutMs: SESSION_READY_TIMEOUT_MS,
+  })
+  return false
+}