Fix sidecar: advance refs/heads/main after each commit

post-receive uses `git checkout -f <SHA>` which detaches HEAD in the bare
repo. Without this fix, sidecar commits advance the detached HEAD but not
refs/heads/main and are orphaned on the next push.

Also commits CLAUDE.md docs update and the retry-loop sync-vps.sh.

edit/server.py

@@ -133,6 +133,16 @@ async def _git_commit(file_path: Path, handle: str, verb: str) -> None:
                 stderr=asyncio.subprocess.DEVNULL,
             )
             await commit.wait()
+            # post-receive uses `git checkout -f <SHA>` which detaches HEAD, so
+            # commits would advance the detached HEAD but not refs/heads/main and
+            # would be lost on the next push. Explicitly keep main up to date.
+            update_ref = await asyncio.create_subprocess_exec(
+                "git", "update-ref", "refs/heads/main", "HEAD",
+                cwd=str(_ROOT), env=env,
+                stdout=asyncio.subprocess.DEVNULL,
+                stderr=asyncio.subprocess.DEVNULL,
+            )
+            await update_ref.wait()
     except Exception as e:
         print(f"[git] commit failed for {rel}: {e}", flush=True)