- Create internal/configfile package for config.json handling
- bd init now creates .beads/config.json with database, version, and jsonl_export fields
- Database discovery checks config.json first, falls back to beads.db
- Update .gitignore to not ignore config.json (part of repo state)
- Update test to expect beads.db and config.json
- Backward compatible with existing beads.db-only setups
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