Vendoring ledger: state the a8bc06ae pin as done; retire the inventory items upstream absorbed

Every vendor/intx row and the sidecar fork now carry the 2026-10-26 kill
date, VENDORED.md describes the current pin in one paragraph instead of
the v0.3.0 release notes plus a re-pin-in-progress note, and
docs/revendor-inventory.md marks CL-6164 (empty-mail drop) and CL-6326
(onBodyFailure) retired with the upstream and workbench commits that
closed them. The sidecar's signing-keypair.ts is byte-identical to
upstream again and atomic-write.ts differs only by the repo-wide logger
namespace; the ledger row says so.

Author: TheGreatAxios