Add XTTS support to Makefile and create CLAUDE.md guide

Makefile improvements:
- Add voices-xtts target to download speaker samples
- Add test-xtts target for testing HD model
- Split voices into voices-piper and voices-xtts
- Update help text with all new targets

speech.py:
- Fix threading import scope issue for XTTS
- Remove redundant 'import threading' inside Piper block

docs/CLAUDE.md:
- Complete guide for Claude Code contributors
- Makefile-first development philosophy
- Never create dirs manually, always use Makefile
- Documentation requirements and testing philosophy
- Common mistakes to avoid
- Raccoon mission values and principles

This ensures consistent, repeatable deployments and makes it easy
to add new TTS engines following the same pattern.

🦝 Generated with [Claude Code](https://claude.com/claude-code)

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

speech.py

@@ -243,7 +243,6 @@ async def generate_speech(request: GenerateSpeechRequest):
 
         # Log any stderr output from Piper
         if tts_proc.stderr:
-            import threading
             def log_stderr():
                 stderr_output = tts_proc.stderr.read().decode('utf-8', errors='replace')
                 if stderr_output.strip():