Expose thread APIs for message board access (#59154)

## What changed

- Add `CodexThread` methods for reading the registration source, resolving agent references, and reading the configured clock.
- Replace the board-specific thread lookup with public `ThreadManager::get_thread_with_extension`, which requires matching typed extension data and never loads a missing runtime.
- Update the local message board host to use these APIs and thread startup metadata instead of accessing session internals directly.

GitOrigin-RevId: 143b798cc710e079e20be59a84d05738a3bc6a0b

codex-rs/core/src/codex_thread.rs

@@ -245,6 +245,36 @@ impl CodexThread {
         self.submit_with_trace(op, /*trace*/ None).await
     }
 
+    /// Returns the source used for registration and agent-reference resolution.
+    /// Internal runtimes can have a different source in their captured session settings.
+    pub fn session_source(&self) -> &SessionSource {
+        &self.session_source
+    }
+
+    /// Resolves an agent reference through this thread's controller without loading it.
+    pub async fn resolve_agent_reference(&self, target: &str) -> CodexResult<ThreadId> {
+        self.session
+            .services
+            .agent_control
+            .resolve(
+                self.session.thread_id,
+                self.session_source.parent_thread_id(),
+                &self.session_source,
+                target,
+            )
+            .await
+    }
+
+    /// Reads this thread's configured clock, including an external or simulated clock.
+    pub async fn current_time(&self) -> CodexResult<DateTime<Utc>> {
+        self.session
+            .services
+            .time_provider
+            .current_time(self.session.thread_id)
+            .await
+            .map_err(|error| CodexErr::Io(std::io::Error::other(error)))
+    }
+
     /// Returns the session telemetry handle for thread-scoped production instrumentation.
     pub fn session_telemetry(&self) -> SessionTelemetry {
         self.session.services.session_telemetry.clone()