fix(slice4): make all entry write paths sidecar-aware (§22.4a carried from SLICE-1)

graduate, claim, retire/unretire, _read_meta_entry, mark_entry_reviewed,
body extract/wrap (api_branches + api_prs replay) now dual-read and write
metadata to the sidecar via write_entry_files + bot.commit_entry_files/
open_entry_pr — a migrated body-only .md no longer crashes entry.parse or
re-grows frontmatter; legacy entries lazy-migrate on first metadata write.
Existing tests updated to assert the sidecar (INV-2 clean docs).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/tests/test_retire_vertical.py

@@ -57,12 +57,15 @@ def test_rfc_owner_can_retire_and_entry_leaves_every_surface(app_with_fake_gitea
         assert r.status_code == 200, r.text
         assert r.json()["state"] == "retired"
 
-        # Meta entry on main: state retired, body + fields kept.
-        meta = entry_mod.parse(
-            fake.files[("wiggleverse", "meta", "main", "rfcs/ohm.md")]["content"]
+        # §22.4a SLICE-4: the state flip lands in the metadata sidecar and the
+        # `.md` is lazy-migrated to a clean body-only file (INV-2). Fields kept.
+        import yaml as _yaml
+        sc = _yaml.safe_load(
+            fake.files[("wiggleverse", "meta", "main", "rfcs/ohm.meta.yaml")]["content"]
         )
-        assert meta.state == "retired"
-        assert "carol" in meta.owners
+        assert sc["state"] == "retired"
+        assert "carol" in sc["owners"]
+        assert "---" not in fake.files[("wiggleverse", "meta", "main", "rfcs/ohm.md")]["content"]
 
         # Cache flipped; gone from the catalog.
         cached = db.conn().execute(
@@ -177,11 +180,12 @@ def test_site_owner_can_retire_active_and_unretire_restores_active_with_id(app_w
         assert r.status_code == 200, r.text
         assert r.json()["state"] == "active"
 
-        meta = entry_mod.parse(
-            fake.files[("wiggleverse", "meta", "main", "rfcs/ohm.md")]["content"]
+        import yaml as _yaml
+        sc = _yaml.safe_load(
+            fake.files[("wiggleverse", "meta", "main", "rfcs/ohm.meta.yaml")]["content"]
         )
-        assert meta.state == "active"
-        assert meta.id == "RFC-0042"
+        assert sc["state"] == "active"
+        assert sc["id"] == "RFC-0042"
 
         cached = db.conn().execute(
             "SELECT state, rfc_id FROM cached_rfcs WHERE slug = 'ohm'"