feat: add Gemini CLI extension support (closes #3)

[federicobartoli]

Closes #3.

Summary

Make this repo installable as a Gemini CLI extension with a single command, while keeping skills/ as the single source of truth shared with Claude Code. No files are copied, no installer script, no wrapper skills.

gemini extensions install https://github.com/addyosmani/web-quality-skills

What this adds

• gemini-extension.json — extension manifest at the repo root. Gemini CLI auto-discovers the existing skills/ directory; no explicit path field is needed.
• .gemini/skills — git-tracked symlink to ../skills (9 bytes, mode 120000). This is the symlink suggested in Add skills in Gemini #3: it enables workspace mode so Gemini CLI auto-loads the same skills when launched from a clone of this repo, without going through gemini extensions install.
• docs/gemini-setup.md — install, workspace mode, and Windows fallback.
• README.md — Gemini CLI install section under Quick start.

Net change: 4 files, ~69 lines, 0 duplicated skill content.

Why no skills field in gemini-extension.json

Per the Gemini CLI extension reference, the skills/ subdirectory is a fixed convention — Gemini CLI auto-discovers skills/<name>/SKILL.md from the extension root. Since this repo already has skills/ at the root, the manifest stays minimal.

Why the .gemini/skills symlink

Issue #3 specifically asked: "Wonder if you can synlink the skills to different folders e.g. .gemini/skills". Gemini CLI also looks at .gemini/skills/ for workspace skills, which load automatically when Gemini CLI runs inside the repo. The git-tracked symlink wires that up out of the box, no copy required.

Compatibility

Gemini CLI and Claude Code share the same SKILL.md format (YAML frontmatter with name + description). The existing 6 skills work as-is on both platforms.

Verified

Tested locally with Gemini CLI:

• gemini extensions install /path/to/local/clone succeeds.
• All 6 skills auto-discovered:
  • web-quality-audit
  • performance
  • core-web-vitals
  • accessibility
  • seo
  • best-practices
• Workspace mode also works: launching gemini from inside the repo picks up the symlinked skills.

Relation to other PRs on this repo

• #10 — Codex plugin support (same no-duplication approach).
• #11 — Claude Code plugin support.

Same intent across the three PRs: one skills/ directory, three install paths, zero duplication.

Test plan

• gemini extensions install /path/to/local/clone succeeds
• All 6 skills auto-discovered after install
• Workspace mode loads .gemini/skills when running gemini from the repo root
• Skills auto-activate on relevant prompts after install (pending maintainer verification)