[FEATURE]: Allow desktop prompt deeplinks to use the current workspace #34762

Feature hasn't been suggested before.\n\n- [x] I have verified this feature I'm about to request hasn't been suggested before.\n\n### Describe the enhancement you want to request\n\nOpenCode Desktop currently supports prompt-prefill links with an explicit project directory:\n\ntext\nopencode://new-session?directory=<absolute-path>&prompt=<url-encoded prompt>\n\n\nThat works well for externally addressable project links, but it makes prompt-only integrations awkward. Other desktop agent launchers can pass just a prompt and let the already-open app decide the active workspace. OpenCode could support the same ergonomic path:\n\ntext\nopencode://new-session?prompt=<url-encoded prompt>\n\n\nRequested behavior:\n\n- If Desktop is running in a local workspace, open a new session in the current initialized workspace and prefill the prompt.\n- If Desktop is running but no local workspace is active, fall back to the existing behavior of ignoring the link or opening a safe default route without submitting anything.\n- Keep the current explicit directory behavior unchanged for deterministic project links.\n- Do not auto-submit the prompt. The existing safety property from the prompt deeplink should remain intact.\n\nWhy this helps:\n\n- Makes OpenCode consistent with prompt-only deeplink integrations where callers do not know the user's local filesystem path.\n- Avoids requiring browser or web components to know an absolute project path.\n- Keeps deterministic project links available for callers that do have a directory.