fix(tmux-subagent): wait for session readiness before spawning attach pane

`opencode attach` was invoked inside a freshly-split tmux pane before the
child session appeared in the opencode server's status map.  The process
exited immediately (session not found), tmux auto-closed the pane, and the
subagent ran invisibly in the background — the race documented in #3505.

Fix: call `waitForSessionReady` *before* `executeActions` in
`session-created-handler.ts`, mirroring the guard already present in
`TmuxSessionManager.ensureSessionReadyBeforeSpawn()`.  If the session does
not become attachable within the timeout the handler returns early without
spawning a pane at all, eliminating the transient-pane and silent-close
failure modes.  The now-unreachable post-spawn readiness-check / pane-close
cleanup branch is removed.

Adds a regression test suite (session-created-handler.test.ts) covering:
- not-ready session → no pane spawned, no polling started
- duplicate session.created → idempotent
- non session.created event type → no action
- already-tracked session → idempotent

Closes #3505

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/features/tmux-subagent/session-created-handler.ts

@@ -102,6 +102,19 @@ export async function handleSessionCreated(
       return
     }
 
+    // Wait for the child session to be registered in the opencode server's status
+    // map BEFORE spawning the tmux pane. If we spawn first, `opencode attach`
+    // exits immediately (session not yet visible), tmux auto-closes the pane, and
+    // the subagent runs invisibly in the background — the bug described in #3505.
+    const sessionReady = await deps.waitForSessionReady(sessionId)
+    if (!sessionReady) {
+      log("[tmux-session-manager] session readiness failed before spawn", {
+        sessionId,
+        stage: "session.created",
+      })
+      return
+    }
+
     const result = await executeActions(decision.actions, {
       config: deps.tmuxConfig,
       directory: deps.directory,
@@ -137,26 +150,6 @@ export async function handleSessionCreated(
       return
     }
 
-    const sessionReady = await deps.waitForSessionReady(sessionId)
-    if (!sessionReady) {
-      log("[tmux-session-manager] session not ready after timeout, closing spawned pane", {
-        sessionId,
-        paneId: result.spawnedPaneId,
-      })
-
-      await executeActions(
-        [{ type: "close", paneId: result.spawnedPaneId, sessionId }],
-        {
-          config: deps.tmuxConfig,
-          directory: deps.directory,
-          serverUrl: deps.serverUrl,
-          windowState: state,
-        },
-      )
-
-      return
-    }
-
     deps.sessions.set(
       sessionId,
       createTrackedSession({