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

[potiuk]

Goal: Bring Google's Gemini CLI to feature parity with Claude Code as a skill runtime. Grounds: RFC-AI-0004 Principle 3 — Vendor neutrality, 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/<name>/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
• 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/

[maheshsv]

I will work this issue