Merge feat/gemini-cli-extension: Add Gemini CLI extension support

Adds Adspirer as a Gemini CLI extension with remote MCP server,
custom commands, model context, GitHub Actions release workflow,
and template-generated skills. Includes OAuth workaround for
known Gemini CLI bug (google-gemini/gemini-cli#20017).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>