feat(#775): ship a gemini-extension.json extension package (#818)

Add a Gemini CLI extension package so users can install, update, and
remove GSD through Gemini's own extension lifecycle and have it appear in
`gemini extensions list`:

  gemini extensions install https://github.com/open-gsd/gsd-core
  gemini extensions update gsd-core
  gemini extensions uninstall gsd-core
  gemini extensions link /path/to/gsd-core   # dev

This mirrors the additive Claude Code plugin manifest (#766): a thin,
version-stamped manifest enforced by an in-repo drift test. The extension
ships the context-file payload (GEMINI.md), loaded into every Gemini
session; slash-command/agent/hook TOML projection into the extension is a
documented follow-up. The manual `npx gsd-core --gemini` installer (which
provides the /gsd:* commands) is unchanged — purely additive, no breaking
change.

- gemini-extension.json: name=binName, version tracks package.json,
  description, contextFileName=GEMINI.md (minimal; no mcpServers — gsd
  ships no MCP server)
- GEMINI.md: Gemini-session context payload
- package.json: add both artifacts to files[] so they publish
- CONTEXT.md: add "Gemini Extension Package" glossary entry
- docs: USER-GUIDE + install-on-your-runtime how-to
- tests/issue-775-gemini-extension.test.cjs: manifest validity, version
  parity with package.json, contextFileName existence, files[] publication

Closes #775

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

.changeset/gemini-extension-package.md

@@ -0,0 +1,5 @@
+---
+type: Added
+pr: 775
+---
+**Gemini CLI extension package** — gsd-core now ships a `gemini-extension.json` manifest (plus a `GEMINI.md` context payload) at the repository root, so Gemini CLI users can install, update, and remove GSD through Gemini's own extension lifecycle: `gemini extensions install https://github.com/open-gsd/gsd-core`, `gemini extensions update gsd-core`, `gemini extensions uninstall gsd-core`, and `gemini extensions link <path>` for local dev. The extension is discoverable in `gemini extensions list` and loads GSD's operating context into every session. Additive — the existing `npx gsd-core --gemini` installer (which provides the `/gsd:*` slash commands) is unchanged. (#775)