Add a feature for a fallback behavior if a project leaf dir is moved as part of an automated process

[travelingflwr]

What happened

A thread whose workspace folder is a leaf directory (C:\Users\xxxxx\Claude_Work\cases\xxxxxxxxx) became unopenable after that folder was legitimately deleted as part of the work the thread was doing:

This thread's workspace folder no longer exists or is not a directory:
C:\Users\xxxxxx\Claude_Work\cases\xxxxxxxxx.
Restore the folder at this path before retrying.

The thread had no other fault — full history intact, work complete. Recovery required manually recreating the deleted directory by hand; T3 does not fall back, prompt, or self-heal. The folder has to be restored exactly where the user just deliberately removed it, which also silently undoes the cleanup the thread existed to perform.

Why this isn't an edge case

The workspace folder being deleted is a normal outcome of some tasks. Here the thread's job was to archive a support case, which moves cases/<id>/ to cases/Closed/<id>/. The thread doing its job correctly is what breaks the thread.

It also can't complete that job unaided: on Windows the workspace folder appears to be held open for the thread's lifetime, so shutil.move / Remove-Item on it fails with WinError 32 — the process cannot access the file because it is being used by another process. Deleting it from an outside shell succeeds, and then the thread can't reopen.

Repro

1. Start a thread with workspace <some>/<leaf-dir>
2. From an external shell, delete <leaf-dir>
3. Reopen the thread → error above; thread is unusable until the folder is manually recreated at that exact path

Expected

One of:

• Fall back to the nearest existing ancestor directory, with a notice ("workspace X no longer exists; using Y")
• Prompt to re-point the thread at a different folder
• At minimum, let the thread open read-only so the transcript is reachable

Hard-failing on a missing directory makes the transcript unreachable at exactly the moment it's most useful — right after the operation that removed the folder.

Environment: Windows 11 Enterprise 22631, T3 Code, Claude Code harness