dsh-plugin-conversation-delete: Permanently delete conversation in DSH

By default, DSH archiving does not remove sessions from the disk. The `dsh-plugin-conversation-delete` plugin provides a confirmed permanent deletion option in the menu of each conversation, cleaning up sessions logs, `session_projcache` projection cache, and sidebar entries. It is rejected when there is an active turn or a background task. It requires DSH >= 0.2.2-rc.1 and Web UI, MIT 0.1.0. Installation is available via `dsh plugin add dsh-plugin-conversation-delete` (not yet published on npm) or a local tgz. Deletion is irreversible; the API is only callable by the owner via browser.