feat: migrate add-on from Gemini CLI to Antigravity CLI (agy) (#10)

[gplasky]

Fixes #10

Summary of Changes

This PR updates the Home Assistant add-on to migrate from the sunsetting Gemini CLI to Google's official Antigravity CLI (agy).

🚀 Key Improvements

1. CLI Migration & Docker Build:

   • Replaced global npm package @google/gemini-cli with Google's official Antigravity CLI installer (curl -fsSL https://antigravity.google/cli/install.sh | bash).
   • Deprecated build.yaml in favor of inlining multi-arch BUILD_FROM and OCI metadata labels directly into Dockerfile.
   • Ensured reliable path resolution for agy in /usr/local/bin/agy.

2. Backward Compatibility:

   • Added /usr/local/bin/gemini alias script to route legacy gemini commands smoothly to agy.
   • Retained support for GEMINI_API_KEY environment variables alongside ANTIGRAVITY_API_KEY.
   • Maintained full compatibility for existing GEMINI.md context files.

3. MCP Configuration:

   • Migrated Home Assistant MCP server configuration out of settings.json into standalone mcp_config.json as required by agy.

4. Branding & Assets Refresh:

   • Updated add-on text branding from "Gemini Terminal" to "Antigravity CLI".
   • Updated add-on icon and logo (icon.png and logo.png) with official Antigravity full-color branding.

5. Automated Testing:

   • Added a Python unit test suite (tests/test_config_and_scripts.py) to validate shell script syntax (bash -n), configuration structure, fallback aliases, and installation logic.

Verification

• Unit test suite executed cleanly (python3 -m unittest discover -s tests).
• Verified build and execution on Home Assistant instance.