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/

tests/antigravity/run-tests.sh

@@ -0,0 +1,16 @@
+#!/usr/bin/env bash
+# Run all Antigravity (agy) integration tests.
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+
+echo "=== Antigravity integration tests ==="
+
+for t in "$SCRIPT_DIR"/test-*.sh; do
+  echo
+  echo ">>> $t"
+  bash "$t"
+done
+
+echo
+echo "=== All Antigravity tests passed ==="