Make parent-owned sub-agent threads read-only in the TUI (#33841)

## Why

Multi-agent V2 spawned threads are controlled by their parent and reject direct
app-server input. The TUI should expose that ownership instead of presenting an
input flow that the server will reject.

## What changed

- Add the experimental `Thread.canAcceptDirectInput` capability for loaded
  threads and populate it from the same policy used to validate `turn/start`
  and `turn/steer`. Stored, unloaded threads report `null`.
- Carry the capability through TUI thread discovery, selection, startup, and
  resume flows, with a compatibility fallback for older app servers.
- Present parent-owned threads as view-only, preserve drafts and queued input,
  block agent-directed commands and settings shortcuts, and keep local
  navigation and inspection commands available.

## Testing

- Cover V1 writable and V2 view-only spawned threads through selection and
  resume.
- Cover composer submission paths, queued and initial input preservation,
  allowed navigation commands, settings shortcuts, and the view-only UI.

GitOrigin-RevId: 495ad051249e5577fb0b0c1cc8c40a0998d04918

codex-rs/tui/src/chatwidget.rs

@@ -202,6 +202,8 @@ const CONNECTORS_SELECTION_VIEW_ID: &str = "connectors-selection";
 const PET_SELECTION_LOADING_VIEW_ID: &str = "pet-selection-loading";
 const AMBIENT_PET_WRAP_GAP_COLUMNS: u16 = 2;
 const TUI_STUB_MESSAGE: &str = "Not available in TUI yet.";
+const PARENT_OWNED_INPUT_MESSAGE: &str =
+    "This sub-agent is controlled by its parent. Direct input is disabled.";
 
 /// Choose the keybinding used to edit the most-recently queued message.
 ///
@@ -650,6 +652,7 @@ pub(crate) struct ChatWidget {
     thread_name: Option<String>,
     thread_rename_block_message: Option<String>,
     active_side_conversation: bool,
+    blocks_direct_input: bool,
     normal_placeholder_text: String,
     side_placeholder_text: String,
     forked_from: Option<ThreadId>,
@@ -1732,6 +1735,15 @@ impl ChatWidget {
         T: Into<AppCommand>,
     {
         let op: AppCommand = op.into();
+        if self.blocks_direct_input
+            && matches!(
+                &op,
+                AppCommand::UserTurn { .. } | AppCommand::Review { .. } | AppCommand::Compact
+            )
+        {
+            self.add_error_message(PARENT_OWNED_INPUT_MESSAGE.to_string());
+            return false;
+        }
         self.prepare_local_op_submission(&op);
         if op.is_review() && !self.bottom_pane.is_task_running() {
             self.bottom_pane.set_task_running(/*running*/ true);