fix: IsNull resolves the uppercase YAML nulls Null and NULL

[ethanhawkes-gif]

Summary

frontmatter.IsNull (and its byte-identical lint-side copy isNull) treated null as only ""/"null"/"~", missing the YAML 1.1 (!!null) / YAML 1.2 core-schema (§10.2.1.1) spellings Null and NULL — the same set the memory scalar parser (internal/core/memory/yaml.go) already resolves. Closes #290.

Why it matters

On the lifeboat pack path gvSupersededADRs unquotes superseded_by and then asks IsNull, so a live (status: accepted) ADR carrying superseded_by: NULL in a foreign target repo was silently emitted as a superseded-adr finding into graveyard/abandoned.json, quoting the null literal as its evidence — a false "this decision was abandoned" claim in a machine-read artefact that graveyard_lessons.go and the graveyard-interpreter agent later cite.

Fix

• Widen both predicate copies together (frontmatter.IsNull and lint.isNull) to the four YAML nulls + empty scalar, so the two gates capture/validate.go depends on cannot drift. isAbsentValue inherits it (it delegates to isNull).
• Correct the IsNull doc comment, which invoked "the YAML nulls" but enumerated three of four.
• Fields does not strip quotes, so a quoted "NULL" still reaches IsNull as a string in the general path and stays non-null; only the pre-existing lifeboat unquote path reads a bare NULL as null — the intended reading for a handle-or-null field, matching the matrix in frontmatter.IsNull misses the YAML nulls Null/NULL, so disembark pack reports a live ADR as superseded — quoting superseded_by: NULL as its evidence #290.

Tests (fail before, pass after)

• frontmatter.TestIsNull / lint.TestIsNull — assert the four spellings null; reject near-misses (None, nil, NUL, nullish) and quoted forms.
• lint.TestIsAbsentValueUppercaseNull — an uppercase null is an absence.
• lifeboat.TestAbandonedAcceptedADRWithUppercaseNullIsNotReported — end-to-end over gvSupersededADRs; walks the full frontmatter.IsNull misses the YAML nulls Null/NULL, so disembark pack reports a live ADR as superseded — quoting superseded_by: NULL as its evidence #290 matrix (null | Null | NULL | ~ | "NULL" | 'Null' → no finding) with a positive control (superseded_by: adr-9 → still reported).

Verification (go1.25.6, matching go.mod)

go test ./... and go test -race ./... green; go vet ./... clean; gofmt -l clean. A CHANGELOG ### Fixed entry accompanies the change.

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Disclosure: this change is AI-assisted; the commit carries Assisted-by: Claude:claude-opus-4-8 per CONTRIBUTING, with a human Signed-off-by (DCO). Happy to adjust anything to fit your conventions.

[REPPL]

Thank you for this — the widening itself is correct, the tests are green, and as our first outside contribution it also pressure-tested our conventions in useful ways. We commissioned a deep review of the diff; the full evidence trail now lives in the repo under .abcd/work/reviews/2026-08-19-pr-294-null-predicate/. The verdict is sound in scope, with a handful of changes we'd like before merge — all small, and all inside what this PR already touches:

1. Delegate instead of duplicating (internal/core/lint/lint.go, isNull): the widened copy is byte-identical to frontmatter.IsNull, and lint.go already imports frontmatter (it delegates its scanner for exactly this reason). Please replace the body with return frontmatter.IsNull(v) and drop the duplicated table test — that removes the very drift class this fix exists to prevent, and makes the "mirrored tests" question moot.

2. Doc comment count (internal/core/frontmatter/frontmatter.go, the IsNull comment): "an empty value and the four YAML nulls ""/"null"/"Null"/"NULL"/"~"" lists the empty scalar twice and five items under "four". The lint-side phrasing ("four YAML nulls (null | Null | NULL | ~) plus the empty scalar") is the correct one.

3. t.Errorf in the spelling matrix (internal/core/lifeboat/graveyard_abandoned_test.go): t.Fatalf inside the loop stops at the first failing spelling and the positive control never runs; t.Errorf (or a subtest per spelling) reports the whole matrix.

4. Widen the capture-side agreement test (internal/core/capture/parse_test.go, TestValidateStrictImpact): it still iterates only {"", "null", "~"} — please add the uppercase spellings so the package whose acceptance changed also pins it.

5. CHANGELOG citation: our changelog cites ledger handles rather than GitHub numbers. We've retroactively captured the bug as iss-287 — please cite (iss-287) instead of (#290).

One thing we are deliberately not asking you to fix here: the review found that quoted nulls (impact: "NULL") are judged differently by capture (which unquotes before the predicate) and record-lint (which reads the raw value). That predates your change, needs a maintainer design decision, and is now tracked as iss-285 with a follow-up intent (itd-128) to consolidate the scalar decoders — your PR's evidence made that class visible, which is genuinely valuable.

Happy to merge once the five points above land.

Assisted-by: Claude:claude-fable-5