feat: add native support for Antigravity

[ludovicc]

Description

This PR introduces native support for the Antigravity agent platform to the arckit-gemini repository. By adding standard plugin metadata and translating CLI-centric tools to native Antigravity APIs, this allows the extension to function natively as an Antigravity plugin out of the box. Antigravity is the successor of Gemini CLI.

Key Changes

1. Plugin Metadata Scaffolding:

   • Added plugin.json in the root directory to define the plugin name, version, description, and keywords.
   • Added installed_version.json specifying the current release version 5.3.0.

2. Subagent Prompt Tool Compatibility:

   • Updated the 10 autonomous subagents inside agents/ (arckit-research, arckit-datascout, cloud-specific research agents, etc.) to use Antigravity's native tools:
     • WebSearch -> search_web
     • WebFetch -> read_url_content
     • Write tool / Write -> write_to_file / write_to_file tool

3. Automated Multi-Environment Installer:

   • Added scripts/bash/install-antigravity.sh to fully automate the installation process for both standard CLI (~/.gemini/config/) and IDE (~/.gemini/antigravity-ide/) environments, symlinking plugins/skills and adding the 'arckit' config to extension-enablement.json.

Verification

• Verified that all 10 subagent markdown prompts were correctly edited and tool references translated.
• Validated that plugin.json and installed_version.json adhere to standard Antigravity structures.
• Git status is verified as clean and staged.
• Verified that it works in my Antigravity CLI and IDE.