Add to undo the last applied diff

[that-github-user]

Problem

thinktank apply patches the working tree directly. If the applied changes break something or the user changes their mind, there is no thinktank rollback — the user must manually run git checkout or git restore . and figure out which files were changed.

This is especially painful when:

• The diff touches many files
• The user already staged other changes they don't want to lose
• They applied agent 2 but now want to try agent 3 instead

Proposed Solution

Before applying a diff, save a restore point:

• Record the list of files to be patched
• Store the pre-apply state of each file in .thinktank/restore-<timestamp>.json

Add a thinktank rollback command:

$ thinktank rollback
Restoring 4 files to pre-apply state...
  ✓ src/parser.ts
  ✓ src/index.ts
  ✓ tests/parser.test.ts
  ✓ package.json
Rollback complete. Working tree restored to state before last apply.

Only the most recent apply is tracked (not a full undo stack). Rollback is only available until the next apply or run.

Acceptance Criteria

• apply saves a restore snapshot to .thinktank/restore-<timestamp>.json before patching
• thinktank rollback restores patched files to their pre-apply content
• If no restore point exists, rollback prints a clear error: "No restore point found. Nothing to roll back."
• Rollback works even if the user has made additional edits after apply (only files touched by apply are restored)
• Restore point file is excluded from .gitignore entries (already covered by .thinktank/)
• --preview mode does not create a restore point (no changes were made)
• Restore file is included in the existing 0o600 permission policy

[that-github-user]

Fixed in #113. Added thinktank undo to reverse last apply. Saves diff to .thinktank/last-applied.patch. Generated by thinktank Opus (Copeland +3).