chore: remove unused dependencies (#996)

## Summary
- Remove `@types/fs-extra` - no fs-extra imports found in the codebase
- Remove `mdast-util-from-markdown` - not imported or used anywhere

These dependencies were identified as unused via `depcheck` and
confirmed by searching for imports.

## Test plan
- [x] Build succeeds
- [x] All local tests pass (65 tests)

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

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: B4nan

package.json

@@ -123,7 +123,6 @@
         "@types/cors": "^2.8.17",
         "@types/execa": "^2.0.2",
         "@types/express": "^5.0.0",
-        "@types/fs-extra": "^11",
         "@types/is-ci": "^3.0.4",
         "@types/jju": "^1.4.5",
         "@types/js-levenshtein": "^1",
@@ -138,7 +137,6 @@
         "eslint": "^9.25.1",
         "eslint-config-prettier": "^10.1.2",
         "lint-staged": "^16.0.0",
-        "mdast-util-from-markdown": "^2.0.2",
         "mock-stdin": "^1.0.0",
         "prettier": "^3.5.3",
         "tsup": "^8.5.0",