feat(scripts): add update.bat and rollback.bat launchers

Add user-facing launchers for the incremental update system:
- update.bat: one-click update launcher
- rollback.bat: rollback to previous version
- tests/test_update_integration.py: integration tests for the update system
- README.md: add incremental update guide (section 10)
- .gitignore: add .update_cache/ entry

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

.gitignore

@@ -32,3 +32,7 @@ Thumbs.db
 
 # OpenCode state
 .omo/
+
+# Update cache (auto-generated by build_update.py)
+.update_cache/
+RUNTIME_CHANGED.txt