bridge: replace the mirror PR when a GitHub PR is retargeted to another base

The mirror kept its creation-time base forever. eternitas #167 was stacked
on fix/one-hallway, retargeted to main after #166 merged; ci.yml
(pull_request: branches [main]) then silently never ran for it, while
unfiltered workflows did. An edited event triggers nothing, so close the
stale mirror and open a fresh one on the new base (runs CI at once).
An unknown base is left alone, never guessed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

api/tests/test_pr_status_bridge.py

@@ -390,3 +390,24 @@ def test_ci_hygiene_posts_under_its_own_context(fake, monkeypatch):
     bridge.post_ci_hygiene("windy-chat", SHA, "main", True)
     # the compute-guard status with the same description must not suppress it
     assert [(p["context"], p["description"]) for p in f.posted] == [("windy-git/ci-hygiene", "WARN 1")]
+
+
+def test_retargeted_pr_gets_a_fresh_mirror_on_the_new_base(fake):
+    # eternitas #167: stacked on fix/one-hallway, retargeted to main on GitHub.
+    gh = [{"number": 167, "title": "feat", "html_url": "u",
+           "head": {"ref": "feat/x", "sha": SHA, "repo": {"full_name": f"{bridge.GH_OWNER}/eternitas"}},
+           "base": {"ref": "main"}}]
+    wg = [{"number": 9, "title": "[GH#167] feat", "base": {"ref": "fix/one-hallway"}}]
+    f = fake(gh_prs=gh, wg_prs=wg)
+    assert bridge.sync_prs("eternitas") == [SHA]
+    assert f.closed == [f"/repos/{bridge.WG_OWNER}/eternitas/pulls/9"]
+    assert [(o["base"], o["head"]) for o in f.opened] == [("main", "feat/x")]
+
+
+def test_unchanged_base_leaves_the_mirror_alone(fake):
+    gh = [{"number": 5, "title": "t", "html_url": "u",
+           "head": {"ref": "b", "sha": SHA, "repo": {"full_name": f"{bridge.GH_OWNER}/windy-chat"}},
+           "base": {"ref": "main"}}]
+    f = fake(gh_prs=gh, wg_prs=[{"number": 3, "title": "[GH#5] t", "base": {"ref": "main"}}])
+    bridge.sync_prs("windy-chat")
+    assert f.closed == [] and f.opened == []

scripts/pr_status_bridge.py

@@ -202,7 +202,17 @@ def sync_prs(repo: str) -> list[str]:
         wanted.add(tag)
         heads.append(pr["head"]["sha"])
         if tag in ours:
+            cur = (ours[tag].get("base") or {}).get("ref")
+            if cur is None or cur == pr["base"]["ref"]:  # unknown base: never guess, leave it
                 continue
+            # Retargeted on GitHub (e.g. a stacked PR moved to main after its
+            # parent merged). The mirror kept the OLD base, so workflows filtered
+            # on the base (`pull_request: branches: [main]`) silently stopped
+            # running: eternitas #167, 09-23. Replace the mirror: an "edited"
+            # event triggers nothing, a freshly opened PR runs CI at once.
+            gitea("PATCH", f"/repos/{WG_OWNER}/{repo}/pulls/{ours[tag]['number']}", {"state": "closed"})
+            print(f"  {repo}: GH#{pr['number']} retargeted {cur} -> "
+                  f"{pr['base']['ref']}: replacing its mirror PR")
         st, _ = gitea(
             "POST",
             f"/repos/{WG_OWNER}/{repo}/pulls",