refactor: remove the runtime binary binding; every release registration path verifies the CI signature [CI]

The hash a node reports about itself cannot be trusted (a swapped binary can
copy the old hash), so the runtime binding only caught files that were replaced
by something that did not lie. Provenance stays where it can be verified: the
signed release manifest (digest) and the signed SHA256SUMS.

- Remove BinaryBindingService, the node_bindings table (schema v15 drops it),
  BinaryPolicy, the handshake hash/manifest fields and the admin binary column.
- Drop the per-platform "binaries" list from the release manifest, the hash
  scripts and the updater's installed.manifest copy.
- ReleaseService.PublishAsync now verifies the release signature, so the admin
  POST /releases route is no longer an unsigned path; tests sign with a test key.
- Docs: section 16 explains why the runtime check is gone.

ci/release-node-images.sh

@@ -117,8 +117,6 @@ for component in agent probe; do
   # 没有配置签名密钥（例如 fork）就只发镜像、不发清单。
   if [[ -n "${RELEASE_SIGNING_KEY:-}" ]]; then
     digest="$(cat "$work/$component.digest")"
-    # 清单里的 binaries：所有平台、所有架构的发布压缩包里的可执行文件（不只是镜像里的 musl 那两个），节点握手时上报的哈希对得上其中任何一个
-    binaries="$(NETRC="$netrc" GITEA_BASE="$gitea_base" bash "$root/ci/release-binary-hashes.sh" "$component" "$version")"
-    RELEASE_BINARIES="$binaries" bash "$root/ci/publish-release-offer.sh" "$component" "$image_root/moshiro-$component" "$digest" "$tag" release "$work/$component-release"
+    bash "$root/ci/publish-release-offer.sh" "$component" "$image_root/moshiro-$component" "$digest" "$tag" release "$work/$component-release"
   fi
 done