feat: add first-class stacked PR support to author workflow (#484)

Normal author work is unchanged: gitea_lock_issue still requires the worktree
to be base-equivalent to master/main/dev, and gitea_create_pr still targets a
normal base branch. A *stacked* PR (based on another unmerged PR's branch) is a
new explicit, proof-backed path — it never bypasses the issue lock.

- stacked_pr_support.py: pure policy. Approve a non-master base only when it is
  declared AND owned by a live OPEN PR whose number matches; reject arbitrary,
  mismatched, or stale (merged/closed) branches; require the PR body to document
  the stack (base branch, stacked-on PR, merge ordering).
- issue_lock_worktree.py: read_worktree_git_state / _find_matching_base_ref gain
  an opt-in extra_bases arg so an approved stacked base can anchor
  base-equivalence in addition to master/main/dev (empty by default = unchanged).
- gitea_mcp_server.py:
  - gitea_lock_issue gains stacked_base_branch + stacked_base_pr. When declared,
    it verifies the open PR owns the branch, anchors base-equivalence to it, and
    records approved_stacked_base = {branch, pr_number, verified_open} on the lock.
  - gitea_create_pr validates a non-master base against the lock's approved
    stacked base, re-checks the dependency PR is still open, and enforces the
    stacked-PR body fields. Master-based path untouched.
- Docs: llm-workflow-runbooks.md and work-issue.md document when stacked PRs are
  allowed and the required PR-body wording.

Tests: test_stacked_pr_support.py (18 policy cases), stacked base-equivalence in
test_issue_lock_worktree.py, approved_stacked_base persistence round-trip in
test_issue_lock_store.py. Existing lock/create_pr regressions still pass.

The #482 -> #479/#478 case motivates this: create_pr previously could not open a
stacked PR because the lock required a master-equivalent worktree.

Closes #484.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

tests/test_issue_lock_store.py

@@ -157,6 +157,30 @@ class TestIssueLockStore(unittest.TestCase):
         )
         self.assertFalse(ils.has_active_issue_lock("feat/issue-999-other"))
 
+    def test_approved_stacked_base_survives_round_trip(self):
+        # #484: the approved stacked base recorded on the lock must persist so
+        # gitea_create_pr can validate the non-master base at PR time.
+        record = _lock_record(
+            issue_number=482,
+            branch_name="feat/issue-482-skip-stale-request-changes-pr",
+            approved_stacked_base={
+                "branch": "feat/issue-478-mcp-menu-shell",
+                "pr_number": 479,
+                "verified_open": True,
+            },
+        )
+        path = ils.lock_file_path(
+            remote="prgs",
+            org="Scaled-Tech-Consulting",
+            repo="Gitea-Tools",
+            issue_number=482,
+        )
+        ils.save_lock_file(path, record)
+        stored = ils.read_lock_file(path)
+        self.assertEqual(stored["approved_stacked_base"]["branch"], "feat/issue-478-mcp-menu-shell")
+        self.assertEqual(stored["approved_stacked_base"]["pr_number"], 479)
+        self.assertTrue(stored["approved_stacked_base"]["verified_open"])
+
     def test_atomic_write_preserves_unrelated_lock(self):
         path_a = ils.lock_file_path(
             remote="prgs",