style: fix all golangci-lint issues and format code (refs #17)
Fix 380 lint violations across all Go source files including wsl_v5,
nlreturn, noinlineerr, errcheck, funlen, funcorder, tagliatelle,
perfsprint, modernize, revive, gosec, ireturn, mnd, forcetypeassert,
cyclop, and others.
Key changes:
- Split large handler/command functions into smaller methods
- Extract scan helpers for database queries
- Reorder exported/unexported methods per funcorder
- Add sentinel errors in models package
- Use camelCase JSON tags per tagliatelle defaults
- Add package comments
- Fix .gitignore to not exclude cmd/chat-cli directory
b78d526f02