Remove the dead plugin-dispose module and its dedicated test now that V1 plugin migration removed the last production call site. Clean the remaining bootstrap test mocks so src no longer references createPluginDispose.
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
YeonGyu-Kim