feat(hosts): add Antigravity (agy) declarative config and setup registry

[0xshae]

• Host Config (hosts/agy.ts): Added declarative host specification for agy mapping CLI
  command, global root (.gemini/config/skills/gstack), workspace root (.agents/skills/gstack),
  path rewrites, and runtime symlinks.

  • Host Registry (hosts/index.ts): Registered agy in ALL_HOST_CONFIGS and re-exported the
    configuration.
  • Gitignore (.gitignore): Added .agy/ to prevent tracking generated skill files.
  • Documentation (README.md): Updated the supported agents table to include Antigravity and
    --host agy flag usage.
  • Test Suite (test/host-config.test.ts): Updated registered host count test assertion from
    10 to 11.

  Verification

  • Validated host configuration rules with bun run scripts/host-config-export.ts validate.
  • Verified skill doc generation via bun run gen:skill-docs --host all (generated . agy/skills/gstack/* successfully).
  • Verified unit test suite via bun test test/host-config.test.ts (73/73 tests passing).

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[0xshae]

/trunk merge

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[time-attack]

@0xshae Please make setup accept the declared Agy host and complete its runtime installation path. The identity and roots are correct, but installation currently cannot finish.

[KiDDarn]

I have a tested setup/runtime implementation in #2244 that may help with the requested follow-up here.

Relevant reference commits:

• c4c9f68 — setup host selection and plugin installation path
• 6e4ed2b — runtime generation fixes
• 49964ee — installation hardening and benchmark guardrails

The useful safeguards to carry over are:

• derive plugin.json version from VERSION instead of hardcoding it
• generate Agy skill output before installing it
• verify at least one nested SKILL.md exists before reporting success
• copy/link the runtime sidecars that generated skills reference
• cover setup generation and installed-skill presence with offline tests

Please use those commits as reference rather than cherry-picking blindly: #2244 uses .gemini/config/plugins/gstack/skills, while this PR established its own root/ownership direction.

I split unrelated fixes from #2244 into focused drafts #2282 (skill-check) and #2283 (benchmark provider hardening). I am keeping #2244 draft-only as a temporary reference until its unique Agy setup/runtime work is absorbed here, then it can be closed without losing the implementation history.

[0xshae]

@time-attack @KiDDarn Thank you both for the detailed review and the reference implementation!

I've just pushed a new commit that wires the agy host into the setup script, completing the runtime installation path.

Specifically, I modeled the setup logic after the existing opencode implementation and incorporated the safeguards from #2244:

• Installation wired up: agy is now in the --host allowlist, auto-detection, and has its own create_agy_runtime_root() and link_agy_skill_dirs() functions.
• Skill Generation & Linking: The script now runs bun run gen:skill-docs --host agy prior to installation, and explicitly verifies that at least one SKILL.md was generated before linking skills or reporting success.
• Sidecars: The runtime sidecars (bin, browse/dist, ETHOS.md, etc.) are properly symlinked to match the globalSymlinks declared in agy.ts.
• Path note: I kept the .gemini/config/skills/gstack path as established in my original agy.ts config, rather than adopting the .plugins path from the draft PR.
• Tests: Added the agy offline test cases to test/host-config.test.ts to verify the global root paths, sidecar symlinks, and skill generation correctness. All 77 tests are passing.

Let me know if this looks good to merge or if there's anything else you'd like me to tweak