Allow moving existing threads between projects

Problem

Codex desktop supports working across projects and can hand off a thread between Local and Worktree within the same project. However, there does not appear to be a way to move a thread that was started under the wrong project into the correct one.

The current workaround is to open the old thread, ask for a handoff summary, start a new thread in the destination project, paste the summary, and archive the original. That fragments history, search results, and project organization.

Desired behavior

Add a "Move to project..." or "Reassign project..." action for existing threads.

Expected details

• Available from the thread menu or command palette
• Lets the user choose a destination project
• Preserves the thread transcript, title, pin/archive state, and searchability
• Clearly handles workspace state: either move only the conversation metadata or offer a safe handoff path when Git state is involved
• Warns when a thread has project-specific files, worktree state, or sandbox permissions that will not transfer cleanly

Why this helps

It makes project organization more forgiving and keeps long-running Codex work discoverable in the right place without manual summarization or duplicate threads.