chore: install superpowers skill framework

- Add all 12 skills to .claude/skills/
- Add hooks for session-start bootstrap
- Add CLAUDE.md with using-superpowers bootstrap
- Configure settings.json with SessionStart hooks

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

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+# 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 |
+|----------------------|----------------------|
+| Dispatch a subagent (`Subagent (general-purpose):` template) | `invoke_subagent` with a built-in `TypeName` — `self` for full-capability work, `research` for read-only |
+| 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. |
+
+## Task tracking
+
+Antigravity has **no todo 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.