feat: add Antigravity CLI (agy) support

Antigravity (Google's `agy` CLI) installs the existing Superpowers plugin
directly:

    agy plugin install https://github.com/obra/superpowers

agy imports the bundled skills and runs the plugin's SessionStart hook, so
using-superpowers bootstraps from the first message — verified on agy 1.0.3:
a fresh session given "Let's make a react todo list" auto-triggers the
brainstorming skill instead of writing code. agy discovers skills natively
and, having no Skill tool, loads them by reading SKILL.md with view_file.

No scaffold, installer, or generated context file is needed. This adds only:

- README.md: an Antigravity install section + Quickstart link
- skills/using-superpowers/SKILL.md: reference to the agy tool mapping
- skills/using-superpowers/references/antigravity-tools.md: action->tool
  mapping for agy (view_file, write_to_file, invoke_subagent, manage_task,
  and skill loading via view_file on SKILL.md)
- tests/antigravity/: structural test for the tool mapping, mirroring
  tests/pi/

skills/using-superpowers/SKILL.md

@@ -41,7 +41,7 @@ If CLAUDE.md, GEMINI.md, or AGENTS.md says "don't use TDD" and a skill says "alw
 
 ## Platform Adaptation
 
-Skills speak in actions ("dispatch a subagent", "create a todo", "read a file") rather than naming any one runtime's tools. For per-platform tool equivalents and instructions-file conventions, see [claude-code-tools.md](references/claude-code-tools.md), [codex-tools.md](references/codex-tools.md), [copilot-tools.md](references/copilot-tools.md), [gemini-tools.md](references/gemini-tools.md), and [pi-tools.md](references/pi-tools.md). Gemini CLI users get the tool mapping loaded automatically via GEMINI.md.
+Skills speak in actions ("dispatch a subagent", "create a todo", "read a file") rather than naming any one runtime's tools. For per-platform tool equivalents and instructions-file conventions, see [claude-code-tools.md](references/claude-code-tools.md), [codex-tools.md](references/codex-tools.md), [copilot-tools.md](references/copilot-tools.md), [gemini-tools.md](references/gemini-tools.md), [pi-tools.md](references/pi-tools.md), and [antigravity-tools.md](references/antigravity-tools.md). Gemini CLI users get the tool mapping loaded automatically via GEMINI.md.
 
 # Using Skills
 

skills/using-superpowers/references/antigravity-tools.md

@@ -0,0 +1,96 @@
+# Antigravity CLI (`agy`) Tool Mapping
+
+Skills speak in actions ("dispatch a subagent", "create a todo", "read a file"). On the Antigravity CLI (`agy`) these resolve to the tools below.
+
+| Action skills request | Antigravity CLI equivalent |
+|----------------------|----------------------|
+| Read a file | `view_file` |
+| Create a new file | `write_to_file` |
+| Edit a file | `replace_file_content` |
+| Edit a file in several places at once | `multi_replace_file_content` |
+| Run a shell command | `run_command` |
+| Search file contents | `grep_search` |
+| Find files by name / list a directory | `list_dir` (no dedicated glob tool — combine `list_dir` with `grep_search`) |
+| Fetch a URL | `read_url_content` |
+| Search the web | `search_web` |
+| Pose a structured question to your human partner | `ask_question` |
+| Dispatch a subagent (`Subagent (general-purpose):` template) | `invoke_subagent` with a built-in `TypeName` — `self` for full-capability work, `research` for read-only (see [Subagent support](#subagent-support)) |
+| Multiple parallel dispatches | Multiple entries in one `invoke_subagent` call's `Subagents` array |
+| Task tracking ("create a todo", "mark complete") | a **task artifact** — `write_to_file` with `IsArtifact: true` and `ArtifactType: "task"` (see [Task tracking](#task-tracking)). **Not** `manage_task`, which manages background processes. |
+
+## Invoking a skill — read its `SKILL.md`
+
+Antigravity surfaces every installed skill's `name` + `description` to you at the
+start of each session, but it has **no `Skill`/`activate_skill` tool**. To load a
+skill, **read its `SKILL.md` with `view_file`, setting `IsSkillFile: true`** when
+the skill applies — e.g. `view_file` on
+`.../plugins/superpowers/skills/<skill-name>/SKILL.md` with `IsSkillFile: true`.
+(`IsSkillFile` is agy's own signal that you're reading a file to *execute its
+instructions*, not to edit or preview it — set it whenever you load a skill.)
+
+This is the blessed skill-loading mechanism on this harness. The general rule
+"never read skill files manually" means "don't bypass your platform's
+skill-loading mechanism" — and on Antigravity, reading `SKILL.md` *is* that
+mechanism. Reading it honors the rule rather than breaking it.
+
+You already know which skills exist and what they're for: their names and
+descriptions are in front of you at session start. When a description matches
+what you're about to do, read that skill's `SKILL.md` before acting.
+
+## Subagent support
+
+Antigravity dispatches subagents with `invoke_subagent`, passing each one a
+`TypeName` in the `Subagents` array. Two `TypeName`s are **built in** — use them
+directly, no `define_subagent` needed:
+
+- **`self`** — a full clone of you, with every tool you have (including
+  `write_to_file`/`replace_file_content`/`run_command`). The safe default for
+  general-purpose work: implementing, fixing, anything that edits files or runs
+  commands.
+- **`research`** — read-only (file reading, `grep_search`, web/URL fetch; no write
+  or command access). Use it when you specifically want a subagent that can't make
+  changes — investigation and read-only review.
+
+Call `define_subagent` only for a custom system prompt or capability mix: set
+`enable_write_tools: true` to grant file edits **and** `run_command`,
+`enable_subagent_tools` for nested dispatch, `enable_mcp_tools` for MCP. Then
+invoke it by the name you gave it. (`manage_subagents` lists/kills running
+subagents.)
+
+Skills dispatch with `Subagent (general-purpose):` and either reference a
+prompt-template file (e.g. `superpowers:subagent-driven-development`'s
+`./implementer-prompt.md`) or supply an inline prompt. On Antigravity:
+
+| Skill dispatch form | Antigravity equivalent |
+|---------------------|----------------------|
+| An implementer-style `*-prompt.md` template (writes code, runs tests) | Fill the template, then `invoke_subagent` with `TypeName: "self"` and the filled prompt |
+| A read-only reviewer template (`spec-reviewer`, `code-quality-reviewer`, `code-reviewer`, `requesting-code-review`'s `./code-reviewer.md`) | `invoke_subagent` with `TypeName: "research"` and the filled review template |
+| Inline prompt (no template referenced) | `invoke_subagent` with `TypeName: "self"` (or `"research"` if the task only reads) and your inline prompt |
+
+### Prompt filling
+
+Skills provide prompt templates with placeholders like `{WHAT_WAS_IMPLEMENTED}` or
+`[FULL TEXT of task]`. Fill all placeholders before passing the complete prompt to
+`invoke_subagent`. The prompt template itself contains the agent's role, review
+criteria, and expected output format — the subagent will follow it.
+
+### Parallel dispatch
+
+Put multiple entries in a single `invoke_subagent` call's `Subagents` array to run
+independent subagent work in parallel. Keep dependent tasks sequential, but do not
+serialize independent subagent tasks just to preserve a simpler history.
+
+## Task tracking
+
+Antigravity has **no todo / `TodoWrite` tool** (`manage_task` manages background
+processes — `list`/`kill`/`status`/`send_input` — it is *not* a checklist). When a
+skill says to create a todo list or track tasks, maintain a **task artifact**: a
+markdown checklist saved with `write_to_file` (`IsArtifact: true`,
+`ArtifactMetadata.ArtifactType: "task"`), edited with `replace_file_content` /
+`multi_replace_file_content` as you go.
+
+At the start of any multi-step task, create the task artifact listing every step of
+your plan. As you complete each step, edit the artifact to mark it done (`- [x]`).
+If the plan changes, update the checklist. Keep it current — it is your source of
+truth for what remains; once the conversation gets long, re-read it before starting
+each step.