fix: downgrade lock file not found message to debug level

The lock file may have been released already when the job completed,
so not finding it in the cache is expected behavior, not an error.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: bpiwowar

src/experimaestro/locking.py

@@ -901,12 +901,15 @@ def release(self, dependency: "DynamicDependency") -> None:
             lock_key = self._lock_file_key(lock_path)
             lf = self.cache.get(lock_key)
             if lf is None:
-                logging.error(
-                    "Could not find lock file for %s (%s)", dependency, lock_key
+                # Lock file may have been released already (e.g., job completed)
+                logger.debug(
+                    "Lock file not in cache for %s (%s) - may have been released already",
+                    dependency,
+                    lock_key,
                 )
                 return
 
-            logging.debug("Deleting %s from cache", lock_key)
+            logger.debug("Deleting %s from cache", lock_key)
             del self.cache[lock_key]
 
             self._do_release(dependency)