Improve carmack.racket.md with code examples and Racket-specific guidance
Changes:
- Add concrete code examples for match/cond and contract-out
- Add Phase separation section (for-syntax vs runtime)
- Add Continuations section (call/ec over call/cc, parameterize)
- Add Concurrency section (places, threads, channels, sync)
- Add Gotchas section (eq?/equal?/eqv?, null?/empty?, string=?)
- Tighten Packages/tooling (raco pkg install --auto, info.rkt)
Removed generic advice:
- 'Don't swallow exceptions' (obvious)
- 'Add docstrings/comments' (obvious)
- 'Include runnable examples' (obvious)
- 'Optimize the bottleneck only' (obvious)
- Entire 'Output expectations' section (meta, not Racket-specific)
- Removed oddly specific 'file/sha1, file-watch' reference
5ad9fb3718