docs(changeset): answer the ADR-0087 disposition question for #12603

check-adr-0087-registration flagged the changeset as declaring a breaking
change (the BREAKING-CHANGE-footer regex matched an ordinary hard-wrapped
sentence starting "breaking change does not burn a major version...") with
no adr-0087: disposition marker. Whatever tripped the regex, the underlying
verdict is correct on the merits -- this changeset does argue an accept-set
narrowing -- so the fix is to answer the question, not to reword around it.

Category: no-migration-prescription, verified against the real predicates
(scripts/check-adr-0087-registration.mjs: parseChangeset/hasMigrationPrescription)
rather than assumed:
- hasMigrationPrescription(body) is false both before and after adding the
  marker -- the changeset's "**Migration.**" paragraph is prose guidance
  ("use a string key instead"), not a heading, an arrow rewrite, a FROM/TO
  label, or a rewrite table, so it never sets framedSection or matches
  REWRITE_RE.
- runtime-interface-only does not apply and was ruled out mechanically, not
  just judgment: exportedTypeDeclaration only recognizes an exported
  interface/type/class/enum ("const/function are absent on purpose -- this
  category is about a TYPE surface"), and every symbol this diff touches
  (installFlatInput, refuseSymbolPayloadKey) is a private function; the one
  exported function in the file whose behaviour changed (wrapDeclarativeHook)
  had no type-declaration change, only a Proxy trap's runtime behaviour, so
  naming it would be a category the gate's own predicate cannot verify true.

ADR-0087 addendum (2026-08-13) authorizes the category: "no-migration-
prescription -- REFUSED when the changeset's own body carries a migration
prescription" -- ctx.input carries no spec/Zod field, object definition, or
stored representation for objectstack migrate meta to act on; a symbol key
was never a declarable metadata surface in the first place, so there is
nothing here for a migration to rewrite.

Part of #12603

Claude-Session: https://claude.ai/code/session_01LZbWd2jNV1FErXTPSS4Dry

Author: claude

.changeset/hook-input-symbol-key-refusal.md

@@ -56,3 +56,5 @@ Inverts the pin `hook-input-ownkeys-agreement.test.ts` carried OPEN since
 #12578 (the disagreement, deliberately left standing) into a REFUSAL pin
 (the write throws, nothing persists) — the same case, turned around in place,
 not a second assertion stacked beside the first.
+
+<!-- adr-0087: not-required (no-migration-prescription) A JS-runtime Proxy trap now throws instead of silently accepting a write; no spec or Zod field, object definition, or stored representation is added, removed, or renamed. A symbol key was never a declarable metadata surface for objectstack migrate meta to know about, so there is nothing here for a migration to rewrite -- the Migration section above is authoring guidance for hook bodies, not a metadata-ledger prescription, and carries no arrow or FROM/TO pair. -->