fix(sqlfluff): address round-3 review (violations section, early-exit, field aliases, README)

- F1: gate 'No issues found' on success; surface stderr on failure (lint_cmd)
- F2: early_exit_on_failure() so non-zero exit surfaces stderr (sqlfluff_cmd)
- F3: add description field + per-violation section (50-line cap, ruff-style)
- F4: '+N more rules' overflow after top-rules list
- F5: '+N more rules' overflow in per-file breakdown
- F6: dual-field line/col deserialization (start_line_no/line_no aliases)
- F7: README example with bare-path usage
- F8: pointer-only (lockstep comment already in lint_cmd)

Test fixture updated to include realistic sqlfluff fields (end_line_no,
end_line_pos, fixable) that real output always emits.