chore(changeset): declare the ADR-0087 disposition on the isAsync retirement (#6148 gate)

The changeset declares a breaking change (major + bang), so
check-adr-0087-registration requires the ledger question to be answered in
writing. It IS registered: the D3 SemanticMigration
`action-descriptor-is-async-retired` plus the exact-key
RETIRED_KEYS_BY_MAJOR[17] entry both land in this PR.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011sGk4SKHqGRgmmqUok1P8M

Author: claude

.changeset/action-descriptor-is-async-retired.md

@@ -6,6 +6,8 @@
 
 refactor(spec)!: retire `ActionDescriptor.isAsync` — a second spelling of `supportsPause` that nothing ever read (#6748, ADR-0049)
 
+<!-- adr-0087: registered action-descriptor-is-async-retired -->
+
 **FROM → TO:** `isAsync: true` → delete the key; declare `supportsPause: true` (plus the
 `resumeAuthority` its pauses need) and return `suspend: true` from `execute()`.
 `isAsync: false` → delete the key; there was never anything to preserve.