Allow codex thread start in plan mode

Additional use case: parent-thread orchestration

The same capability is also needed when one Codex task creates and coordinates another Codex task.

Today, the Desktop agent-facing create_thread tool requires an initial prompt and starts the new task immediately, but it does not expose collaborationMode. send_message_to_thread also cannot change the target task's collaboration mode. As a result, a parent task cannot create a new task whose first turn runs in native Plan mode; prompt text such as "plan only" can imitate the behavior, but it is not the real Plan mode.

I verified locally that the underlying app-server already supports the required lifecycle:

thread/start
→ thread/settings/update (collaborationMode.mode = "plan")
→ turn/start

The missing piece is exposing this through the Desktop task-management tools. Either of these API shapes would solve the orchestration case:

create_thread({
  prompt: "...",
  collaborationMode: "plan",
  target: ...
})

or:

const child = create_thread_empty(...)
set_thread_settings({
  threadId: child.threadId,
  collaborationMode: "plan"
})
send_message_to_thread({ threadId: child.threadId, prompt: "..." })

Expected behavior:

• A parent Codex task can create another visible task and select native Plan mode for it.
• The child task's first turn uses the built-in Plan-mode instructions and tools; no Default-mode turn starts first.
• The selected mode is visible in the child task and persists for subsequent turns until changed.
• Existing callers that omit the mode retain the current Default behavior.

This is closely related to the proposed codex://threads/new?mode=plan deep-link parameter, but covers agent-to-agent/task orchestration in addition to external-app integration.