feat(session-recovery): add filesystem-based empty content recovery

- Replace API-based recovery with direct JSON file editing for empty content messages
- Add cross-platform storage path support via xdg-basedir (Linux/macOS/Windows)
- Inject '(interrupted)' text part to fix messages with only thinking/meta blocks
- Update README docs with detailed session recovery scenarios

package.json

@@ -46,6 +46,7 @@
     "@ast-grep/napi": "^0.40.0",
     "@code-yeongyu/comment-checker": "^0.4.1",
     "@opencode-ai/plugin": "^1.0.7",
+    "xdg-basedir": "^5.1.0",
     "zod": "^4.1.8"
   },
   "devDependencies": {