feat(hosts): add Google Antigravity (agy) host support

[ManchalaSashank]

• Add hosts/antigravity.ts declarative configuration with agy CLI and Gemini model overlay

  • Register antigravity in hosts/index.ts and ALL_HOST_CONFIGS
  • Add setup installer support (--host antigravity / --host agy) targeting ~/.gemini/config/skills
  • Add Antigravity cleanup support to bin/gstack-uninstall
  • Update test/host-config.test.ts assertions for 11 hosts
  • Update README.md Other AI Agents matrix

  Why

  Currently, developers using the Google Antigravity CLI (agy / antigravity) have no native host target in gstack. Running ./setup defaults to
  Claude Code (~/.claude/skills/), which Antigravity does not discover since it looks for global skills in ~/.gemini/config/skills/ and workspace
  rules in AGENTS.md/GEMINI.md. This change integrates Antigravity as a first-class host following gstack's declarative defineHost architecture,
  enabling automated skill generation, setup installation with --host antigravity (or --host agy), and uninstallation.

  Live evidence

  Ran host validation, registry checks, and alias resolution:

  $ npx tsx test-antigravity-host.ts
  === Host Verification ===
  Total hosts registered: 11
  Host names: claude, codex, factory, kiro, opencode, slate, cursor, openclaw, hermes, gbrain, antigravity
  
  Antigravity config: {
    "name": "antigravity",
    "displayName": "Google Antigravity",
    "cliCommand": "agy",
    "cliAliases": [
      "agy"
    ],
    "defaultModel": "gemini",
    "globalRoot": ".gemini/config/skills/gstack",
    "localSkillRoot": ".agents/skills/gstack",
    "hostSubdir": ".gemini",
    "usesEnvVars": true,
    "frontmatter": {
      "mode": "allowlist",
      "keepFields": [
        "name",
        "description"
      ],
      "descriptionLimit": null
    },
    "generation": {
      "generateMetadata": false,
      "skipSkills": []
    },
    "pathRewrites": [
      {
        "from": "~/.claude/skills/gstack",
        "to": "~/.gemini/config/skills/gstack"
      },
      {
        "from": ".claude/skills/gstack",
        "to": ".agents/skills/gstack"
      },
      {
        "from": ".claude/skills",
        "to": ".gemini/skills"
      },
      {
        "from": "CLAUDE.md",
        "to": "AGENTS.md"
      },
      {
        "from": "~/.codex/skills/gstack",
        "to": "~/.gemini/config/skills/gstack"
      },
      {
        "from": ".codex/skills",
        "to": ".agents/skills"
      }
    ],
    "suppressedResolvers": [
      "GBRAIN_CONTEXT_LOAD",
      "GBRAIN_SAVE_RESULTS"
    ],
    "runtimeRoot": {
      "globalSymlinks": [
        "bin",
        "browse/dist",
        "browse/bin",
        "gstack-upgrade",
        "ETHOS.md"
      ],
      "globalFiles": {
        "review": [
          "checklist.md",
          "TODOS-format.md"
        ]
      }
    },
    "install": {
      "linkingStrategy": "symlink-generated"
    },
    "coAuthorTrailer": "Co-Authored-By: Google Antigravity <antigravity@google.com>",
    "learningsMode": "basic"
  }
  
  Validation errors: None (Valid!)
  
  Alias resolution test:
  resolveHostArg("agy") -> antigravity
  resolveHostArg("antigravity") -> antigravity
  

Scope

• Changed: hosts/antigravity.ts, hosts/index.ts, setup, bin/gstack-uninstall, test/host-config.test.ts, README.md.
• Verified live by: Tested host schema validation with validateHostConfig(), alias resolution for agy/antigravity, and registry mapping.
• Did NOT test: Live full-suite model evals against paid Gemini API quota in CI.

Liveness proof (required for external contributors)

Checklist

[✓] Liveness screenshot attached (GSTACK PR typed live into a real surface)
[✓] This is not a generated-file-only diff (edited the host config source and setup script)
[✓] No ETHOS.md edits, and no changes to voice / founder perspective / YC references
[✓] New public command / external service / host adapter has an accepted issue linked (or N/A)
[ ] Linked issue or reproduction: #1977

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