Use app server metadata for fork parent titles (#18632)

## Problem
The TUI resolved fork parent titles from local CODEX_HOME metadata,
which could show missing or stale titles when app-server metadata is
authoritative.

This is a lingering bug left over from the migration of the TUI to the
app-server interface. I found it when I asked Codex to review all places
where the TUI code was still directly accessing the local CODEX_HOME.

## Solution
Route fork parent title metadata through the app-server session state
and render only that supplied title, with focused snapshot coverage for
stale local metadata.

## Testing
I manually tested by renaming a thread then forking it and confirming
that the "forked from" message indicated the parent thread's name.

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

@@ -0,0 +1,52 @@
+use super::App;
+use crate::app_server_session::ThreadSessionState;
+use crate::read_session_model;
+use codex_app_server_protocol::Thread;
+use codex_protocol::ThreadId;
+
+impl App {
+    pub(super) async fn session_state_for_thread_read(
+        &self,
+        thread_id: ThreadId,
+        thread: &Thread,
+    ) -> ThreadSessionState {
+        let mut session = self
+            .primary_session_configured
+            .clone()
+            .unwrap_or(ThreadSessionState {
+                thread_id,
+                forked_from_id: None,
+                fork_parent_title: None,
+                thread_name: None,
+                model: self.chat_widget.current_model().to_string(),
+                model_provider_id: self.config.model_provider_id.clone(),
+                service_tier: self.chat_widget.current_service_tier(),
+                approval_policy: self.config.permissions.approval_policy.value(),
+                approvals_reviewer: self.config.approvals_reviewer,
+                sandbox_policy: self.config.permissions.sandbox_policy.get().clone(),
+                cwd: thread.cwd.clone(),
+                instruction_source_paths: Vec::new(),
+                reasoning_effort: self.chat_widget.current_reasoning_effort(),
+                history_log_id: 0,
+                history_entry_count: 0,
+                network_proxy: None,
+                rollout_path: thread.path.clone(),
+            });
+        session.thread_id = thread_id;
+        session.thread_name = thread.name.clone();
+        session.model_provider_id = thread.model_provider.clone();
+        session.cwd = thread.cwd.clone();
+        session.instruction_source_paths = Vec::new();
+        session.rollout_path = thread.path.clone();
+        if let Some(model) =
+            read_session_model(&self.config, thread_id, thread.path.as_deref()).await
+        {
+            session.model = model;
+        } else if thread.path.is_some() {
+            session.model.clear();
+        }
+        session.history_log_id = 0;
+        session.history_entry_count = 0;
+        session
+    }
+}