feat: add Gemini CLI extension support

[ikarenkov]

Description

This PR adds native support for Gemini CLI, enabling users to install the Obsidian skills collection as a functional extension.

Key changes:

• Added gemini-extension.json: The mandatory manifest file that allows Gemini CLI to recognize the repository as an extension and automatically discover all bundled skills in the skills/ directory.
• Updated README.md: Added clear installation instructions for Gemini CLI users, including the --auto-update flag for seamless synchronization with the repository.
• Added .gitignore: Included standard exclusions (e.g., .DS_Store, node_modules/) to maintain repository cleanliness across different agent environments.

How to test this PR:

You can install and verify the extension directly from this branch using the following command:

gemini extensions install https://github.com/ikarenkov/obsidian-skills --ref feat/gemini-support

Note: When prompted to "attempt to install via 'git clone' instead", please answer Y.

Example of installation flow:

Test results:

• ✅ Installation: The extension installs successfully via git clone fallback.
• ✅ Discovery: Gemini CLI correctly identifies and lists the extension.
• ✅ Skills Registration: All bundled skills (obsidian-markdown, obsidian-bases, json-canvas, obsidian-cli, defuddle) are automatically discovered and available.
• ✅ Functionality: Verified that the agent correctly activates the relevant skills when prompted with Obsidian-specific tasks.

[ikarenkov]

@kepano hi! Is there any chance to merge it?