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/app-server-protocol/src/protocol/common.rs

@@ -2639,6 +2639,7 @@ mod tests {
                     cwd: cwd.clone(),
                     cli_version: "0.0.0".to_string(),
                     source: v2::SessionSource::Exec,
+                    can_accept_direct_input: None,
                     thread_source: None,
                     agent_nickname: None,
                     agent_role: None,
@@ -2692,6 +2693,7 @@ mod tests {
                         "cwd": absolute_path_string("tmp"),
                         "cliVersion": "0.0.0",
                         "source": "exec",
+                        "canAcceptDirectInput": null,
                         "threadSource": null,
                         "agentNickname": null,
                         "agentRole": null,