feat(shared): add plugin identity constants for package name migration

Add centralized plugin identity constants to support migration from
oh-my-opencode to oh-my-openagent. Includes both current and legacy
names for backward compatibility.

🤖 Generated with assistance of OhMyOpenCode

src/shared/index.ts

@@ -64,3 +64,4 @@ export * from "./prompt-tools"
 export * from "./internal-initiator-marker"
 export * from "./plugin-command-discovery"
 export { SessionCategoryRegistry } from "./session-category-registry"
+export * from "./plugin-identity"