feat: cut a release with abcd launch ship and a derived changelog

[REPPL]

Phase 3 of the derived-versioning + auto-changelog + plugin-distribution programme
(plan: .abcd/development/plans/2026-07-21-derived-version-and-changelog.md; spec: spc-11).

Fourth of five stacked PRs. Based on feat/itd-73-p2-surface-guardrail (#127).

The write-verb the whole programme hangs on did not exist: launch was dry-run-only and
Ship() stopped at WouldPublish. Verified before building:
abcd launch ship → unknown command "ship", abcd changelog → unknown command.

It runs, on this repo, today

$ go run ./cmd/abcd changelog
abcd changelog — REFUSED (base v0.3.0)
  decided by: iss-51
  guard:      refused
  added:      17 record(s)
    [breaking ] iss-51   context-scoped-successor-bans
    [additive ] iss-47   cli-reference-generation
    [internal ] iss-97   detect-unbounded-marker-reads  (excluded from the changelog)
    …
  refused (surface-guard):
    no surface baseline in v0.3.0 … so a break cannot be detected and the cut refuses
    rather than passing unguarded.

changelog exits 0 (a status render). launch ship exits 1 (the cut refuses).
CHANGELOG.md is untouched by both. The refusal is the phase-2 fail-closed baseline rule
firing correctly — it clears when a release is cut that contains the baseline.

Two entry points, mirroring disembark

abcd launch ship with no flag is the deterministic emit step. With
--changelog-json <file|-> it ingests the host-composed payload, runs the bijection and
writes. The host runs the composer between them — the same shape as
disembark … --principles-json.

The completeness bijection

Cited record ids must equal the cut's records minus the internal ones. No omission, no
invention. This is deliberately not SynthesizePrinciples' cite-or-be-dropped semantic:
an omitted record is a lie by omission in a durable release record, so a mismatch refuses the
whole payload and writes nothing, the way ComposePressRelease refuses an uncited
document. The payload guards do come from the principles path (size cap, unknown-field
rejection, sanitisation).

The agent controls wording and the Keep-a-Changelog section only. Version and inclusion
are deterministic.

Refusals — six kinds, all fail-closed

no-release-tag, release-in-flight, unlabelled-record, stale-intent, surface-guard,
empty-cut. Typed rather than prose-matched, so a caller acts on the kind instead of
pattern-matching English. A refused cut carries no derived version — refusing while still
emitting a number invites someone to use it.

stale-intent is the itd-94 case: an intent still in planned/ whose spec is closed is
invisible to the tree diff, so the cut would silently under-bump. It now refuses and names it.

What the two independent reviews caught — both FIX_FIRST, both fixed

1. The heading could have been written where the release workflow would never see it.
   insertSection asserted the new heading's shape but never its position. In a
   CHANGELOG.md whose ## [Unreleased] anchor sits below an older dated section, the new
   heading lands below an already-tagged one — and auto-release.yml's grep -m1 matches the
   older heading. The cut reports success and the release is silently never tagged.
   Now the assembled bytes are scanned and the write refuses unless the new heading is the
   first dated one, naming the heading the workflow would otherwise tag.
2. Both reviewers independently found the bijection had a hole. A record on the cut's
   removed side whose blob at the base tag carries no valid impact was dropped from the
   required set — so a supersession would never reach the release record. The security
   reviewer added the sharper half: an honest composer that cited it would then be refused
   for "inventing" it. Fixed at the canonical home (Record.InChangelog()), so the CLI
   render, the composer prompt's stated rule, and the bijection all read one predicate.

Plus two hardening nits: trailing data after the JSON document is now refused, and an
over-long cited id is described by length rather than echoed.

The composer agent

release-changelog-composer — named distinctly because the CHANGELOG and README agent
slots are occupied by two documents the loader mis-registers as agents (iss-110).
Carries prompt_version: 0.1.0, reads_untrusted_input: true, an injection-canary fixture,
and the untrusted-data contract. Logged in agents/CHANGELOG.md (the agent prompt log — not
the repo changelog).

Reuse over reinvention

The in-flight detection is Derive's, now classified by a typed RefusalKind rather than
reimplemented. The stale-intent check consumes a ScanSpecLinks index extracted from the
lint's existing spec-lifecycle traversal, which was rewritten to consume it — no third walk
of the same trees.

Verification

gofmt -l . silent · go vet ./... · go build ./... · go test ./... ·
go run ./cmd/record-lint exit 0 · both new verbs exercised on the real repo.

Tested under CI's actual condition before pushing: the check job checks out with no
tags, which is what turned #125 and #127 red. The full suite passes in a clone with its tags
deleted. That fix is merged forward into this branch.

CHANGELOG.md, go.mod, go.sum and both .github/workflows/*.yml are untouched —
ADR-37's release contract is preserved, which is the entire point of writing the heading the
existing grep already matches.