* feat(commands): add safe git revert command /gsd-undo
Adds a new /gsd-undo command for safely reverting GSD phase or plan
commits. Uses phase manifest lookup with git log fallback, atomic
single-commit reverts via git revert --no-commit, dependency checking
with user confirmation, and structured revert commit messages including
a user-provided reason.
Three modes: --last N (interactive selection), --phase NN (full phase
revert), --plan NN-MM (single plan revert).
Closes#1730
* fix(commands): address review feedback for /gsd-undo
- Add dirty-tree guard before revert operations (security)
- Fix manifest schema to use manifest.phases[N].commits (critical)
- Extend dependency check to MODE=plan for intra-phase deps
- Handle mid-sequence conflict cleanup with reset HEAD + restore
- Fix unbalanced grep alternation pattern for phase scope matching
- Remove Write from allowed-tools (never needed)
