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/templates/start-issue.md

@@ -26,8 +26,19 @@ Steps:
    cd branches/<type>-issue-<n>-<slug>
 6. Implement the narrow scope only; add/update focused tests if behavior changes.
 7. Checks: run the test suite, compile/lint changed files, git diff --check,
-   and scan the diff for secrets.
-8. Commit (issue-linked message), push the branch, open a PR to master.
+   and scan the diff for secrets. Record the branch name and HEAD SHA at
+   validation time.
+8. Branch proof before commit (#177) — prove and state:
+   - git branch --show-current == the intended issue branch from step 5
+   - the branch is NOT master/main/develop/development/dev
+   - branch and HEAD unchanged since step 7 (another session can switch a
+     shared checkout mid-session; if drift is detected, STOP and reconcile
+     before committing)
+   If a commit accidentally lands on a protected branch: do NOT push;
+   report the accident and the exact repair steps — never silently continue.
+9. Commit (issue-linked message). Branch proof before push (#177): local
+   branch == push target branch == intended issue branch, none protected.
+   Then push the branch and open a PR to master.
    *The PR body MUST use closing keywords like `Closes #N` or `Fixes #N` to close the issue; do NOT use `Implements #N` or `Refs #N` for closing, as Gitea will not auto-close it.*
    Include an "LLM Handoff Metadata" block in the PR body (attribution only;
    never an eligibility input — docs/llm-agent-sha.md):
@@ -40,7 +51,7 @@ Steps:
    - Branch: <branch>
    - Worktree: <worktree path>
    - Self-review allowed: no
-9. Stop before review/merge — you are the author.
+10. Stop before review/merge — you are the author.
 
 Handoff: issue #, branch, worktree path, files changed, checks + results, PR URL —
 formatted as the compact Controller Handoff (SKILL.md §K; long form only on