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/cli/config-manager/plugin-name-with-version.ts

@@ -1,6 +1,7 @@
+import { PLUGIN_NAME } from "../../shared"
 import { fetchNpmDistTags } from "./npm-dist-tags"
 
-const DEFAULT_PACKAGE_NAME = "oh-my-opencode"
+const DEFAULT_PACKAGE_NAME = PLUGIN_NAME
 const PRIORITIZED_TAGS = ["latest", "beta", "next"] as const
 
 function getFallbackEntry(version: string, packageName: string): string {