[skill request] /agy — Antigravity CLI wrapper as a peer second-opinion tool (like /codex)

## Summary

Google released **Antigravity CLI** (`agy`), a Gemini-backed coding agent CLI that mirrors the Claude / Codex CLI shape (`-p` print mode, `-c` continue, `--add-dir`, plugin system, sandbox mode). It would slot naturally next to `/codex` as a third independent voice for review / challenge / consult.

Please consider adding a `/agy` skill — same three-mode shape as `/codex`:

- **`/agy review`** — independent diff review against the base branch, pass/fail gate
- **`/agy challenge`** — adversarial second opinion that tries to break the change
- **`/agy consult`** — open-ended Q&A with session continuity (`agy -c`)

The pitch: `/codex` already gives users one external voice. Adding `/agy` makes the second-opinion pattern provider-diverse (OpenAI + Google), which is exactly the value of asking outside the family — diverging training data, diverging failure modes.

## Why now

`agy` v1.0.0 is shipped and the print-mode contract is stable enough to wrap:

```
$ agy --version
1.0.0

$ agy -p "你叫什麼名字？你是什麼模型？"
我是 Antigravity，是一個由 Google DeepMind 團隊所開發、
基於 Gemini 3.5 Flash 模型的 AI 程式設計助手。
```

`-p`, `--print-timeout`, `-c`, `--add-dir`, `--dangerously-skip-permissions`, `--sandbox`, and a `plugin` subcommand are all already present (`agy --help`), so the wrapper surface is roughly 1:1 with the existing `/codex` skill.

## Suggested implementation notes

- Mirror `~/.claude/skills/codex/SKILL.md` layout — same preamble, same review/challenge/consult split.
- Use `agy -p` + `--print-timeout` for one-shot review/challenge; use `agy -c` for the consult follow-up loop.
- Pipe the diff via `--add-dir` (so `agy` sees the working tree) rather than stdin-stuffing the prompt, to keep large diffs out of the prompt window.
- Default model is fine — but expose a way to override via env var (`AGY_MODEL` or similar) for users who want Gemini 3.5 Pro vs Flash.
- Detect missing CLI gracefully, same as `/codex` does for `codex`.

## Related

- `/codex` skill — the obvious template
- #350 (closed, not-planned) was about running gstack *on* Antigravity (host-side). This request is the opposite direction: gstack *calling* `agy` as a peer tool, the same way it calls `codex` today.

Happy to help test if useful.

Start by reading ~/.claude/skills/codex/SKILL.md and inspecting agy --help, especially print mode, continuation, directory access, and timeout options. Add the analogous /agy skill with review, challenge, and consult modes, graceful handling for a missing CLI, and model override support; done means the three workflows operate as described.

Written by the indexing model from the issue text.