Fix tui_app_server ghost subagent entries in /agent (#16110)

Fixes #16092

The app-server-backed TUI could accumulate ghost subagent entries in
`/agent` after resume/backfill flows. Some of those rows were no longer
live according to the backend, but still appeared selectable in the
picker and could open as blank threads.

*Cause*
Unlike the legacy tui behavior, tui_app_server was creating local
picker/replay state for subagents discovered through metadata refresh
and loaded-thread backfill, even when no real local session or
transcript had been attached. That let stale ids survive in the picker
as if they were replayable threads.

*Fix*
Stop creating empty local thread channels during subagent metadata
hydration and loaded-thread backfill.
When opening /agent, prune metadata-only entries that thread/read
reports as terminally unavailable.
When selecting a discovered subagent that is still live but not yet
locally attached, materialize a real local session on demand from
thread/read instead of falling back to an empty replay state.

codex-rs/tui/src/app/agent_navigation.rs

@@ -122,6 +122,16 @@ impl AgentNavigationState {
         self.order.clear();
     }
 
+    /// Removes a tracked thread entirely from picker metadata and traversal order.
+    ///
+    /// This is reserved for entries that were only discovered opportunistically and never became
+    /// replayable local threads. Keeping those around after the backend confirms they are gone
+    /// would leave ghost rows in `/agent`.
+    pub(crate) fn remove(&mut self, thread_id: ThreadId) {
+        self.threads.remove(&thread_id);
+        self.order.retain(|candidate| *candidate != thread_id);
+    }
+
     /// Returns whether there is at least one tracked thread other than the primary one.
     ///
     /// `App` uses this to decide whether the picker should be available even when the collaboration
@@ -145,6 +155,14 @@ impl AgentNavigationState {
             .collect()
     }
 
+    /// Returns tracked thread ids in the same stable order used by the picker.
+    pub(crate) fn tracked_thread_ids(&self) -> Vec<ThreadId> {
+        self.ordered_threads()
+            .into_iter()
+            .map(|(thread_id, _)| thread_id)
+            .collect()
+    }
+
     /// Returns the adjacent thread id for keyboard navigation in stable spawn order.
     ///
     /// The caller must pass the thread whose transcript is actually being shown to the user, not