feat(adapter/gemini): Google Gemini CLI skill runtime

**Goal:** Bring Google's Gemini CLI to feature parity with Claude Code as a skill runtime. Grounds: [RFC-AI-0004 Principle 3 — Vendor neutrality](../tree/main/docs/rfcs/RFC-AI-0004.md), which names Gemini CLI by name.

**Why Gemini CLI:** Open-source, Apache 2.0, official Google offering with a growing extension ecosystem. Strong reach into the Google-Cloud and Workspace adopter universe. Gemini CLI also has first-class MCP support, which means many of the framework's MCP-shaped integrations (Gmail, Slack, Google Drive, GitHub) should "just work" once the skill-format adapter lands.

**What "parity" means:**

- Skills under `.claude/skills//SKILL.md` are invokable from Gemini CLI
- The `tools/*` bridges are reachable (language-agnostic — port directly)
- Sandbox parity: Gemini CLI has a `--sandbox` mode; document the mapping to the framework's `bubblewrap` + network-allowlist baseline
- HITL primitives: Gemini CLI's "approve before run" mode is the native fit — verify and document

**Suggested approach:**

- Map `SKILL.md` → Gemini CLI's `.gemini/extensions/` extension format (or `GEMINI.md` for project-level instructions)
- Reuse the framework's MCP servers where Gemini supports them
- Document the skill-invocation pattern: how a maintainer runs a framework skill end-to-end via Gemini
- Map the `setup-isolated-*` sandbox baseline to Gemini's sandbox flags

**Reference:**

- RFC-AI-0004: [`docs/rfcs/RFC-AI-0004.md`](../tree/main/docs/rfcs/RFC-AI-0004.md)
- Gemini CLI: https://github.com/google-gemini/gemini-cli
- Gemini CLI extensions: https://github.com/google-gemini/gemini-cli/blob/main/docs/extension.md
- Existing skill shape: [`.claude/skills/`](../tree/main/.claude/skills/)