feat: improve devcontainer setup and add facets to simple-json output

[mkyutani]

Summary

• Add Poetry and GitHub CLI as devcontainer features (removes manual installation)
• Extend .env.local.sample with GITHUB_TOKEN and ANTHROPIC_API_KEY examples
• Add structured facets field to simple-json output format
• Fix simple-json to return raw text without URL restoration
• Add comprehensive tests for facets extraction
• Update documentation (CLAUDE.md, README.md)

Changes

1. Devcontainer improvements:

   • Use Poetry feature instead of manual curl installation
   • Add GitHub CLI feature
   • Simplify postCreateCommand

2. Simple-json format enhancement:

   • Add facets field with structured arrays for links, mentions, and tags
   • Each facet includes URL/handle/tag, byte positions, and text segment
   • Return raw text (without URL restoration) for better MCP integration

3. Documentation:

   • Update CLAUDE.md with facets architecture details
   • Update README.md with simple-json format examples

Test plan

• All existing tests pass
• New facets tests cover links, mentions, tags, and mixed scenarios
• Devcontainer builds successfully with new features
• simple-json output validated with test suite

Backward Compatibility

• simple-json format change is additive (adds new facets field)
• All other output formats unchanged
• No breaking changes to CLI interface