feat(author-workflow): add fail-closed branch-identity proofs for commit/push (#177)

Adds author_proofs.py, the author-side counterpart of review_proofs.py
(#173), turning the branch-drift incident from PR #176 into fail-closed
gates:

- verify_branch_for_commit: the current branch must equal the intended
  feature branch and may never be a protected branch (master, main,
  develop, development, dev); missing state fails closed.
- detect_branch_drift: any branch or HEAD change between validation time
  and commit time — including an external branch switch in a shared
  worktree, which is treated as an expected event to detect — blocks the
  commit until reconciled.
- verify_push_target: a push requires the local branch, remote target
  branch, and intended issue branch to all match, none protected.
- assess_protected_branch_commit: an accidental protected-branch commit
  must never be pushed, requires repair, and the repair must be reported;
  pushing the accident or silently continuing is a violation.
- build_commit_push_report: final-report block carrying the branch proof
  before commit and before push; any failed proof, drift, or violation
  makes the status blocked.

tests/test_author_proofs.py (27 tests) covers the issue's harness
assertions: commit on master blocked; drift between validation and commit
blocked; push target mismatch blocked; shared-worktree branch switch
detected; repair path cannot silently continue without reporting.

SKILL.md section E and templates/start-issue.md now require recording the
validation-time branch/HEAD, the branch proof before commit, the branch
proof before push, and non-silent accident repair. No review/merge/
permission gate is weakened.

Closes #177

Co-Authored-By: Claude Fable 5 <[email protected]>

skills/llm-project-workflow/SKILL.md

@@ -150,12 +150,33 @@ Worktree folder = branch with `/` replaced by `-`
 6. Implement the narrow scope only — no unrelated refactors or formatting churn.
 7. Add/update focused tests when behavior changes.
 8. Run the checks (tests, compile/lint, `git diff --check`, secret scan).
-9. Commit with an issue-linked message.
-10. Push the branch.
-11. Open a PR to `master`.
-12. **If you are the author, stop before review/merge.**
-13. **Normal issue work must not directly push to `master`.** PR content should be merged through the forge PR merge mechanism.
-14. Direct push to `master` is allowed only as a documented recovery exception. If used, the final report must include:
+   Record the branch name and `HEAD` SHA at validation time — the drift
+   check in step 9 compares against exactly this state.
+9. **Branch proof before commit (#177):** prove and state, immediately
+   before staging/committing (`author_proofs.verify_branch_for_commit`,
+   `author_proofs.detect_branch_drift`):
+   - current branch (`git branch --show-current`) equals the intended
+     feature branch from the issue claim
+   - current branch is not `master`, `main`, `develop`, `development`, or
+     `dev`
+   - branch and `HEAD` have not changed since validation (step 8) — in a
+     shared checkout another session may switch branches mid-session;
+     treat that as expected and **stop before committing** when detected
+   If any check fails, stop and reconcile; do not commit.
+10. Commit with an issue-linked message.
+11. **Branch proof before push (#177):** prove that the local branch, the
+    push target branch, and the intended issue branch all match, and that
+    none of them is a protected branch
+    (`author_proofs.verify_push_target`). If a commit accidentally landed
+    on a protected branch, do **not** push: report the accident and the
+    exact repair steps (`author_proofs.assess_protected_branch_commit`) —
+    never silently continue after a repair.
+12. Push the branch.
+13. Open a PR to `master`. The final report must include the branch proofs
+    from steps 9 and 11 (`author_proofs.build_commit_push_report`).
+14. **If you are the author, stop before review/merge.**
+15. **Normal issue work must not directly push to `master`.** PR content should be merged through the forge PR merge mechanism.
+16. Direct push to `master` is allowed only as a documented recovery exception. If used, the final report must include:
     - why the PR merge path could not be used
     - exact commits pushed
     - PR metadata state