Harden already-merged detection to exact string match

Ganymede review nit: substring match on "already" could false-positive
on future return strings. Pin to the two known values from cherry_pick().

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/merge.py

@@ -1464,7 +1464,7 @@ async def _merge_domain_queue(conn, domain: str) -> tuple[int, int]:
             # Cherry-pick returns "already merged" when all commits are empty.
             # The branch ref still points at old commits (not a descendant of main),
             # so pushing branch_sha:main would fail as non-fast-forward.
-            if "already" in pick_msg.lower():
+            if pick_msg in ("already merged (all commits empty)", "already up to date"):
                 conn.execute(
                     "UPDATE prs SET status = 'merged', merged_at = datetime('now'), last_error = NULL WHERE number = ?",
                     (pr_num,),