Handle moved/renamed base repos and repair existing workspaces

[0xferrous]

Today, repo identity is derived from the path relative to base_repo_dir, and workspace paths mirror that relative path. This works while repos stay in
place, but breaks if a source repo is renamed or moved.

Problem:

• existing workspaces remain tied to the old relative repo path
• discovery sees the moved repo as a different repo identity
• existing Git worktrees break because their metadata points at the old source repo path
• existing JJ workspaces also break when the source repo moves

We should support both proactive renames/moves and retrospective repair.

Proposed direction:

• add a builtin repo rename/move flow that:
  • moves the source repo under base_repo_dir
  • moves the corresponding workspace directories under workspace_dir
  • repairs Git worktree metadata
  • repairs JJ workspace metadata
  • updates agent-box’s view of the repo to the new relative path
• add a repo repair flow for after-the-fact moves/renames, where the source repo has already moved and the managed workspaces need to be
  repaired/relinked

Suggested commands:

• ab repo rename
• ab repo repair

This keeps the initial scope focused on repo-level operations rather than introducing per-workspace repair commands up front.

This issue tracks designing and implementing first-class support for moved/renamed repos instead of treating them as entirely new repos.