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/app/api_prs.py

@@ -23,7 +23,7 @@ from typing import Any
 from fastapi import APIRouter, HTTPException, Request
 from pydantic import BaseModel, Field
 
-from . import auth, cache, chat as chat_layer, db, entry as entry_mod, funder, models_resolver, projects as projects_mod, rfc_links
+from . import auth, cache, chat as chat_layer, db, entry as entry_mod, funder, metadata as metadata_mod, models_resolver, projects as projects_mod, rfc_links
 from .bot import Bot
 from .config import Config
 from .gitea import Gitea, GiteaError
@@ -1009,20 +1009,25 @@ async def _replay_changes(
 
 
 def _extract_body_for_replay(is_super_draft: bool, content: str) -> str:
+    # §22.4a SLICE-4: a meta-resident entry may be legacy (frontmatter+body) or
+    # migrated (body-only). strip_frontmatter handles both without raising.
     if not is_super_draft:
         return content
-    try:
-        return entry_mod.parse(content).body
-    except Exception:
-        return content
+    return metadata_mod.strip_frontmatter(content)
 
 
 def _wrap_body_for_replay(is_super_draft: bool, prior_content: str, new_body: str) -> str:
+    # §22.4a SLICE-4: identity for body-only (migrated) files — never re-grow
+    # frontmatter; preserve a legacy file's frontmatter until its next metadata
+    # edit migrates it.
+    nb = new_body if new_body.endswith("\n") else new_body + "\n"
     if not is_super_draft:
-        return new_body
-    entry = entry_mod.parse(prior_content)
-    entry.body = new_body if new_body.endswith("\n") else new_body + "\n"
-    return entry_mod.serialize(entry)
+        return nb
+    if entry_mod.FRONTMATTER_RE.match(prior_content):
+        entry = entry_mod.parse(prior_content)
+        entry.body = nb
+        return entry_mod.serialize(entry)
+    return nb
 
 
 def _resolution_branch_name(original_branch: str) -> str: