Add UI support to relocate a project folder and move its threads

What problem are you running into?

When a local project folder is moved or renamed, Codex keeps the old project entry in the sidebar and its previous chats/threads remain grouped under the old folder. There does not appear to be a simple UI action to relocate the project or move all existing threads to the new folder.

Example from my setup:

• Old folder: C:\dev\.svelte\menu
• New folder: C:\dev\.svelte\menu-app

After opening the new folder, the sidebar showed both projects. The new menu-app project appeared separately, while older threads still appeared under menu. Moving them required manually updating local Codex metadata, including thread cwd values and persisted sidebar/project state.

What would you like to happen?

Codex should provide a supported UI flow for this, such as:

• Relocate project folder from the project menu
• Move all threads to another project/folder
• Automatic detection when a saved project path no longer exists and a likely replacement folder is opened
• A confirmation dialog showing how many local threads will be moved

Why this matters

Moving or renaming project folders is common. Having the thread history split across stale and new project entries makes it hard to find prior work and makes the sidebar look inconsistent. Manual metadata edits are risky and should not be necessary for normal project maintenance.

Current workaround

Manually updating local Codex state can work, but it is unsupported and easy to miss cached sidebar/project entries. A first-class UI action would be safer and clearer.