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 ==="

tests/antigravity/test-antigravity-tools.sh

@@ -0,0 +1,53 @@
+#!/usr/bin/env bash
+# Validate the Antigravity (agy) integration. agy installs the existing plugin
+# directly (`agy plugin install <repo-url>`): it loads the bundled skills and
+# runs the SessionStart hook for bootstrap, so there is no agy-specific scaffold
+# to test. What IS agy-specific is the tool mapping — agy has no `Skill` tool and
+# loads skills by reading SKILL.md with view_file — and SKILL.md pointing at it.
+#
+# Mirrors tests/pi/test-pi-extension.mjs's "tools reference documents
+# harness-specific mappings" check. CI-safe: does not require `agy` installed.
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+REPO_ROOT="$(cd "$SCRIPT_DIR/../.." && pwd)"
+
+MAPPING="$REPO_ROOT/skills/using-superpowers/references/antigravity-tools.md"
+SKILL="$REPO_ROOT/skills/using-superpowers/SKILL.md"
+
+fail() { echo "FAIL: $*" >&2; exit 1; }
+
+echo "test-antigravity-tools: checking Antigravity tool mapping"
+
+# --- Mapping exists ---------------------------------------------------------
+[ -f "$MAPPING" ] || fail "tool mapping missing at $MAPPING"
+
+# --- Skill-load mechanism: view_file on SKILL.md (IsSkillFile), no Skill tool -
+grep -qiE "view_file" "$MAPPING" \
+  || fail "mapping does not document view_file as the file/skill-read tool"
+grep -qiE "SKILL\.md" "$MAPPING" \
+  || fail "mapping does not document reading SKILL.md as the skill-load path"
+grep -q "IsSkillFile" "$MAPPING" \
+  || fail "mapping does not document setting IsSkillFile when loading a skill"
+
+# --- Core action→tool mappings are documented -------------------------------
+for tool in write_to_file replace_file_content run_command grep_search invoke_subagent; do
+  grep -q "$tool" "$MAPPING" \
+    || fail "mapping does not document the '$tool' tool"
+done
+
+# --- Subagents use the built-in self/research types -------------------------
+grep -q '`self`' "$MAPPING" \
+  || fail "mapping does not document the built-in 'self' subagent type"
+grep -q '`research`' "$MAPPING" \
+  || fail "mapping does not document the built-in 'research' subagent type"
+
+# --- Task tracking documents the 'task' artifact mechanism ------------------
+grep -qE 'ArtifactType.*task|task. artifact' "$MAPPING" \
+  || fail "mapping does not document task tracking as a 'task' artifact"
+
+# --- SKILL.md Platform Adaptation links the mapping -------------------------
+grep -q "antigravity-tools.md" "$SKILL" \
+  || fail "SKILL.md Platform Adaptation does not reference antigravity-tools.md"
+
+echo "PASS: Antigravity tool mapping valid (view_file skill-load, agy tools, SKILL.md link)"