Add Antigravity IDE/CLI support and enhance Windows compatibility

[sunnysingh100]

…rdening

Antigravity integration:

• New hosts/antigravity.ts host config (CLI binary: agy)
• Setup script: auto-detect, runtime root creation, skill linking
• Install to both ~/.gemini/antigravity-ide/ and ~/.gemini/antigravity-cli/
• plugin.json manifest, 47 skills generated with .antigravity/ paths
• Added .antigravity/ to .gitignore

Windows compatibility:

• browser-skill-commands: platform-aware PATH, path.delimiter, execPath
• domain-skills: async/await refactor (replaced callback fs.open pattern)
• xvfb: try/catch around Unix-only ps command
• host-config-export: Bun.which() instead of shell command -v
• server-embedder-terminal-port: import.meta.dir instead of URL.pathname

Test hardening:

• Backslash normalization in path assertions (gen-skill-docs, browser-skills-e2e)
• Platform-aware test skips (context-save, setup-conductor-worktree, artifacts-init, regression-pr1169)
• process.execPath instead of echo in bun-polyfill tests
• Dynamic host detection in host-config detect test
• Updated server-auth sliceBetween markers
• Added migration-checkpoint-ownership to Windows-incompatible list