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/slash_dispatch.rs

@@ -254,6 +254,10 @@ impl ChatWidget {
                 self.submit_user_message(INIT_PROMPT.to_string().into());
             }
             SlashCommand::Compact => {
+                if self.blocks_direct_input {
+                    self.add_error_message(PARENT_OWNED_INPUT_MESSAGE.to_string());
+                    return;
+                }
                 self.clear_token_usage();
                 if !self.bottom_pane.is_task_running() {
                     self.bottom_pane.set_task_running(/*running*/ true);