fix(loop): make Docker sandbox opt-in and preserve progress file

- Add --sandbox flag to loop command (default: use plain claude -p)
- Append to progress.txt instead of overwriting between runs
- Display execution mode (Docker sandbox vs Claude CLI) in output

package.json

@@ -117,7 +117,8 @@
 		"turndown": "^7.2.2",
 		"undici": "^7.16.0",
 		"uuid": "^11.1.0",
-		"zod": "^4.1.12"
+		"zod": "^4.1.12",
+		"proper-lockfile": "^4.1.2"
 	},
 	"optionalDependencies": {
 		"@anthropic-ai/claude-code": "^2.0.59",