refresh CLAUDE.md; add nested guides for tools, exercises, content

Created by: jedmund

Summary

• Root CLAUDE.md rewritten to reflect actual project state: repo layout across synthesize/, content/, lessons/, exercises/, checks/, ear-training/, hardware/; app architecture with entry points, stores, audio, shared KeyboardContainer; docs index. Tightens em dash guidance to a flat "no -- / ---" rule.
• Three new nested CLAUDE.md files so a fresh Claude session can add a tool, exercise, or lesson in one pass without re-exploring:
  • synthesize/synthesize/Views/Tools/: tool registration checklist plus all five Playground parameterization touch-points (PlaygroundConfig, PlaygroundParams, PlaygroundConfigurator, PlaygroundScreen category sets, PlaygroundGrid).
  • synthesize/synthesize/Views/Exercises/: shared shell file-by-file (ExerciseHost, ExerciseState, ScoringOverlay, etc.), ScaleBuilder walkthrough, and new-exercise checklist including the lesson dispatch sites (PageContainer, ActivityStateRegistry).
  • content/: voice and pedagogy playbook, gear-grounded openers, discovery-first pacing, "You can X. Now Y." bridge phrasing, warm-but-not-professor-y register, and a concrete "AI tells to scrub" list.

Test plan

• Read each new file and confirm cited file paths exist (root architecture section, Playground touch-points, dispatch sites).
• Confirm no -- or --- anywhere in the new content.
• Spot-check: does the content/CLAUDE.md voice guidance match how recent lessons read?