feat: add native Antigravity (agy) platform support

[tall27]

Summary

This PR adds native support for Google Antigravity (agy) as a supported AI collaboration platform in Squad.

Key Changes

• Platform Definition (src/setup.rs):
  • Added agy to PLATFORMS with binary agy and global skill discovery path .gemini/config/skills/squad/SKILL.md.
  • Added SQUAD_AGY_CONTENT skill template with YAML frontmatter (name: squad, description) and phase 1/2 workflow instructions.
  • Implemented is_platform_installed(platform) to check PATH for agy and recognize antigravity as an alias.
• CLI & Diagnostics (src/main.rs):
  • Updated squad join to accept --client agy (and --client antigravity alias).
  • Updated squad setup and squad setup --list to detect and list agy.
  • Enhanced squad doctor to support Windows USERPROFILE fallback when HOME is unset.
• Tests & Documentation:
  • Added YAML validity and template generation unit tests in tests/setup_test.rs.
  • Added join integration tests with --client agy and --client antigravity in tests/cli_test.rs.
  • Updated English (README.md) and Chinese (README.zh-CN.md) documentation tables.

Testing

• Ran cargo test --release (152 tests passed).
• Verified squad setup, squad setup --list, and squad join --client agy on Windows.