Add Antigravity, Claude Code, and Codex plugin support with local installer

[dandye]

Description

This PR introduces plugins for Antigravity 2.0 (aka Antigravity Desktop), Antigravity IDE, and Antigravity CLI (aka agy). This new work is alongside the existing Gemini CLI Extension manifest.

The plugins enable developers and users to load the SecOps skills and remote MCP server configuration.

The logic for collecting and configuring the required parameters (i.e. x-goog-user-project, Chronicle Customer ID, etc.) is handled in the interactive installation script (scripts/install.py).

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Key Changes

1. Multi-Agent Manifests

• Added plugin.json at the root for Antigravity plugin discovery.
• Added .claude-plugin/plugin.json and .claude-plugin/marketplace.json for Claude Code plugin namespace and registry configuration.
• Added .codex-plugin/plugin.json for OpenAI Codex skill directory exposure.

2. Antigravity MCP Server Setup

• Created mcp_config.json at the root to declare the remote-mcp-secops server.
• Uses a template pattern allowing parameters (SERVER_URL, PROJECT_ID) to be dynamically configured during installation.

3. Interactive Local Setup

• Created scripts/install.py which:
  • Finds existing parameters from local .env or legacy extension .env files.
  • Interactively prompts the user for configuration with custom defaults (secops-demo-env).
  • Copies the local workspace to the global plugin target (~/.gemini/config/plugins/google-secops).
  • Replaces template placeholders in mcp_config.json with literal values.
  • Prepends configuration parameters into GEMINI.md to establish the environment context for the agent.
• Updated justfile's install-agy recipe to run python3 scripts/install.py.

4. Code Cleanup & Automated Testing

• Removed the legacy setup-antigravity skill which is no longer needed now that the interactive installer is available.
• Created a validation suite (test/run_tests.sh) to ensure:
  • Manifest version numbers remain in sync.
  • Executable files inside skill directories use correct extensions.
  • Skill frontmatter matches expected structure.

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Verification Steps

1. Run the test suite to ensure all validations pass:

   just test

2. Test the installation recipe:

   just install-agy

3. Verify the prompts accept defaults and configure files correctly inside ~/.gemini/config/plugins/google-secops/.