feat(laws): expose the 개정이유 law.go.kr already ships (WI-P3)

[tjdwls101010]

Why

The package could say what a statute now reads (get-law) and what changed between versions (compare-law-versions), but nothing reached why it changed.

HistoryEvent.reason sounds like it should. It doesn't — it carries a bare change-type label, and on 개인정보 보호법 all 13 events return None.

Meanwhile law.go.kr embeds the full 「개정이유 및 주요내용」 in every version detail response at 법령.제개정이유.제개정이유내용, alongside 법령.개정문.개정문내용. Normalization discarded both. get-law was already downloading this text and dropping it on the floor — the fetch was paid for, only the parse was missing.

What

get-revision-reason --law <id> [--mst <mst> | --as-of <date>] → RevisionReason (kind revision_reason_text).

Version selection mirrors how the question actually gets asked:

input	resolves to
--mst	that exact version — the value trace-law-history already puts in each event's identity.mst
--as-of	the version in force on that date
neither	the newest version on file

"Newest" rather than "in force today" is deliberate: the most recent amendment is usually what a 왜 바뀌었나 question means, even when its effective date hasn't arrived. The existing not_effective_as_of flag keeps that from reading as current law — and the acceptance case is itself future-effective, so it does fire.

Two disciplines ride along, because the failure modes here are interpretive rather than technical:

• An 개정이유 is the proposer's own account of intent, not a verified effect. Citing it as the reason a law changed launders an advocacy document into a finding.
• It belongs to one version. Generalizing it into the statute's amendment story silently replaces every other amendment's rationale with this one.

Also

Relaxes a hardcoded == 27 command count in test_refactor_compat.py to >= 27. The invariant worth holding is the parser/catalog set equality asserted directly above it; pinning the number only guarantees that whoever adds a command also edits that line.

Verification

Live against law.go.kr:

• --mst 283839 → "[일부개정] ◇ 개정이유 최근 주요 통신사, 금융사, 플랫폼 사업자 등의 대규모 개인정보 유출사고 …", not_effective_as_of: true
• --as-of 2024-01-01 → mst 248613 (시행 20230915)
• no selector → mst 283839

455 deterministic tests pass (11 new).

🤖 Generated with Claude Code