import-laddr: --limit semantics need a --help doc

\`--limit=N\` truncates per-table to N *imported* rows, but the report still counts pre-limit rows under \`input\`, so \`imported + skipped + errors < input\` when the flag is in play. This is intentional — dry-run reports should reflect the dump's actual size — but may surprise staff scanning a report.

Once we settle on a CLI help framework (in apps/api/scripts/), document:
- \`--limit\` precisely (per-table cap on imported rows; input still counts dump size)
- All the other flags
- The 7-commit pattern + idempotence semantics

Filed as Follow-up from PR #24 (laddr-import).

Start in apps/api/scripts/ and review PR #24 for the laddr-import entry point, existing flags, and the 7-commit and idempotence semantics. After the CLI help framework is settled, document --limit and the remaining flags, with completion marked by help text that accurately explains report counts and import behavior.

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