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

bun.lock

@@ -9,6 +9,7 @@
         "@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": {
@@ -102,6 +103,8 @@
 
     "undici-types": ["undici-types@7.16.0", "", {}, "sha512-Zz+aZWSj8LE6zoxD+xrjh4VfkIG8Ya6LvYkZqtUQGJPZjYl53ypCaUwWqo7eI0x66KBGeRo+mlBEkMSeSZ38Nw=="],
 
+    "xdg-basedir": ["xdg-basedir@5.1.0", "", {}, "sha512-GCPAHLvrIH13+c0SuacwvRYj2SxJXQ4kaVTT5xgL3kPrz56XxkF21IGhjSE1+W0aw7gpBWRGXLCPnPby6lSpmQ=="],
+
     "zod": ["zod@4.1.8", "", {}, "sha512-5R1P+WwQqmmMIEACyzSvo4JXHY5WiAFHRMg+zBZKgKS+Q1viRa0C1hmUKtHltoIFKtIdki3pRxkmpP74jnNYHQ=="],
   }
 }