CLI polish (Cobra) + config file merge (YAML/JSON + env/flags)

## Summary
Adopt Cobra with subcommands and a unified config loader that merges config file, env vars, and flags.

## Motivation
- Cleaner UX
- Reproducible runs and easier ops automation

## Scope
- Commands: `crawl`, `serve`, `resume`, `bench`
- `--config` path; precedence: file < env < flags
- Help text and examples

## Acceptance Criteria
- `gotor crawl -c gotor.yaml` reproduces a run exactly
- `--help` is clear and complete

## Tasks
- [ ] Integrate Cobra
- [ ] Config loader + precedence rules
- [ ] Update README and examples