Add notification hook, directory CLAUDE.md scaffolding

- Wire Notification event in settings.json to new notify.sh hook
- notify.sh: Windows toast notifications via PowerShell balloon tip
- docs/CLAUDE.md: authoring standards for ADRs, TR registry, control manifest, engine reference
- design/CLAUDE.md: GDD directory guidance placeholder
- src/: scaffold with .gitkeep and CLAUDE.md authoring guide

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/CLAUDE.md

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+# Source Directory
+
+When writing or editing game code in this directory, follow these standards.
+
+## Engine Version Warning
+
+The LLM's training data predates the pinned engine version.
+**Always check `docs/engine-reference/` before using any engine API.**
+Do not guess at post-cutoff API signatures — look them up first.
+
+## Coding Standards
+
+- All public APIs require doc comments
+- Gameplay values must be **data-driven** (external config files), never hardcoded
+- Prefer dependency injection over singletons for testability
+- Every new system needs a corresponding ADR in `docs/architecture/`
+- Commits must reference the relevant story ID or design document
+
+## File Routing
+
+Match the engine-specialist agent to the file type being written.
+See `CLAUDE.md` → Technical Preferences → Engine Specialists → File Extension Routing.
+
+When in doubt, use the primary engine specialist configured in `CLAUDE.md`.
+
+## Tests
+
+Tests live in `tests/` — not in `src/`.
+Run `/test-setup` to scaffold the test framework if it doesn't exist yet.
+Every gameplay system should have unit tests covering its formulas and edge cases.
+
+## Verification-Driven Development
+
+Write tests first when adding gameplay systems.
+For UI changes, verify with screenshots.
+Compare expected output to actual output before marking work complete.