File browser: folder upload + copy/move (0.13.0)

Folder upload: the Files page gained an "Upload folder" picker
(webkitdirectory); each file is sent with its webkitRelativePath and the
backend recreates the directory tree. upload_target now accepts an optional
rel_path, creating intermediate dirs (mkdir -p) inside the sandbox with each
component validated against traversal.

Copy/move: new file_service.copy/move + POST /api/files/{copy,move}
(admin, audit-logged). The UI adds per-row copy/cut actions, a clipboard bar
to paste into the current directory, and an overwrite prompt on conflict.
Both refuse to move/copy a folder into itself or its own subtree and are
sandbox-checked on source and destination.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/agent_app.py

@@ -40,7 +40,7 @@ from services import backup_service, compose_service
 
 logger = logging.getLogger("stackpilot.agent")
 
-AGENT_VERSION = "0.12.0"
+AGENT_VERSION = "0.13.0"
 
 
 # --------------------------------------------------------------------------- #