List Gemini output in Gemini CLI extension gallery

Parent: #45

## Goal
Make the generated Gemini output discoverable through Gemini CLI's official extension gallery path.

## Evidence
- Extension gallery: https://geminicli.com/extensions/
- Release/listing docs: https://geminicli.com/docs/extensions/releasing/
- Gemini docs state the gallery automatically indexes public extensions with the `gemini-cli-extension` GitHub topic and a root `gemini-extension.json` manifest.
- Generated output: `plugins/gemini/`.

## Tasks
- Decide whether `plugins/gemini/` needs a standalone publishable repository or export path before gallery discovery.
- Ensure the publishable artifact has a valid root `gemini-extension.json` manifest.
- Add the `gemini-cli-extension` topic to the public repository used for distribution.
- Tag/release as needed and verify it appears in the gallery.
- Record the listing URL/status back in #45.