refactor: restructure chroma-sandbox example for clarity

- Wrap all Chroma logic in ChromaProvider class with clear comments
  explaining why each piece exists (sha256 caching, path translation)
- Show all 4 WarpGrep operations (grep, read, listDir, glob)
- Rename grepTool → searchTool (it does more than grep)
- Clean up remoteCommands wiring to 3 one-liners
- Rewrite README to lead with the pattern, not Chroma details

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>