feat(antigravity): add full support for Google Antigravity CLI agent ecosystem

[makhlwf]

Summary

Adds comprehensive, production-ready support for Google Antigravity CLI (\�gy), Antigravity 2.0, and Antigravity IDE environments across the repository.

This brings Antigravity CLI to 100% agent parity with GitHub Copilot and Claude Code plugins, providing 80 specialized accessibility agents, 26 reusable skills, lifecycle hooks, MCP server configuration, and baseline WCAG 2.2 AA context enforcement.

Changes Included

1. Antigravity Plugin & Extension Bundles

• **.antigravity/\ & \�ntigravity-plugin/**: Dedicated directories containing:
  • 80 Agents: Formatted for \�gy\ subagent selection (/agents\ command).
  • 26 Skills: Domain-specific scanning patterns, scoring, and rule definitions.
  • **\ANTIGRAVITY.md**: Baseline WCAG 2.2 AA enforcement rules and Decision Matrix.
  • **\mcp_config.json**: MCP server integration connecting \�11y-mcp-server\ stdio transport.
  • **\hooks.json\ & \scripts/**: Pre/post tool enforcement hooks (\UserPromptSubmit, \PreToolUse, \PostToolUse).
  • **\plugin.json**: Complete plugin manifest defining identity, rules, hooks, MCP servers, skills, and agents.

2. Installer Enhancements

• PowerShell (\install.ps1): Added -Antigravity\ parameter and automated plugin installation logic to .antigravity\ (project) or ~/.gemini/antigravity-cli/plugins/accessibility-agents\ (global).
• Bash (\install.sh): Added --antigravity\ flag with matching install target resolution.

3. Verification & Validation Scripts

• **\scripts/check-platform-parity.js**: Added Antigravity CLI to parity reporter (80/80 agents, 0 gaps).
• **\scripts/validate-agents.js**: Added \�alidateAntigravityAgent()\ validator (0 errors, 0 warnings).
• **.github/workflows/validate-agents.yml**: Added Antigravity agent paths to CI validation triggers.

4. Comprehensive Documentation

• Created \docs/ANTIGRAVITY-GUIDE.md.
• Updated \README.md, \AGENTS.md, \GEMINI.md, \plugin.yaml, and \docs/getting-started.md.

Quality & Repository Policy Verification

• Strict No Emoji Policy: Verified 100% compliance across all generated, edited, and documentation files (replaced with plain text labels).
• Validation: Passed
  ode scripts/validate-agents.js\ (80 agents, 26 skills, 0 errors).
• Parity Check: Passed
  ode scripts/check-platform-parity.js\ (80/80 agents matched).