feat: add Gemini CLI platform support (MCP config, hooks, workspace skills)

[carlosedsousa-dev]

This PR adds first-class support for the Google Gemini CLI to code-review-graph’s installer.

Key changes:

• New --platform gemini-cli target that configures the MCP server in project scope via .gemini/settings.json.
• Workspace hooks for Gemini CLI:
  • SessionStart shows a short code-review-graph status message.
  • AfterTool (matcher write_file|replace) runs code-review-graph update --skip-flows quietly.
  • Installs hook scripts under .gemini/hooks/ and ensures they are executable.
• Gemini CLI Agent Skills installation:
  • Writes skills to .gemini/skills/<skill>/SKILL.md using YAML frontmatter (name, description).
  • When --platform gemini-cli is used, only Gemini skills are installed (no Claude skills).
• Instruction injection updated so GEMINI.md is owned by both antigravity and gemini-cli.
• Docs/tests:
  • Adds Gemini CLI to supported platforms list and updates install examples.
  • Extends test coverage for Gemini CLI config, hooks, skills, and instruction injection.

How to test:

• uv run pytest -q tests/test_skills.py
• From a repo root: code-review-graph install --platform gemini-cli -y and confirm:
  • .gemini/settings.json contains mcpServers.code-review-graph and hooks entries
  • .gemini/hooks/ scripts exist and are executable
  • .gemini/skills/ contains the generated skills
  • No .claude/skills is created for --platform gemini-cli