feat(compat): package rename compatibility layer for oh-my-opencode → oh-my-openagent

- Add legacy plugin startup warning when oh-my-opencode config detected
- Update CLI installer and TUI installer for new package name
- Split monolithic config-manager.test.ts into focused test modules
- Add plugin config detection tests for legacy name fallback
- Update processed-command-store to use plugin-identity constants
- Add claude-code-plugin-loader discovery test for both config names
- Update chat-params and ultrawork-db tests for plugin identity

Part of #2823

src/shared/index.ts

@@ -70,3 +70,4 @@ export * from "./internal-initiator-marker"
 export * from "./plugin-command-discovery"
 export { SessionCategoryRegistry } from "./session-category-registry"
 export * from "./plugin-identity"
+export * from "./log-legacy-plugin-startup-warning"