feat: Phase 3 - Discovery Endpoints (stat and exists tools)

- Add stat tool for detailed file/folder metadata
- Add exists tool for fast path existence checking
- Add StatResult and ExistsResult type definitions
- Implement stat() and exists() methods in VaultTools
- Register both tools in ToolRegistry with complete schemas
- Update version to 2.1.0
- Update ROADMAP.md to mark Phase 3 complete
- Update CHANGELOG.md with Phase 3 release notes
- Add IMPLEMENTATION_NOTES_PHASE3.md documentation
- Clean up old phase documentation files

versions.json

@@ -1,5 +1,7 @@
 {
 	"1.0.0": "0.15.0",
 	"1.1.0": "0.15.0",
-	"1.2.0": "0.15.0"
+	"1.2.0": "0.15.0",
+	"2.0.0": "0.15.0",
+	"2.1.0": "0.15.0"
 }