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.

backend/src/MoShiro.Backend.Application/Agents/AgentService.cs

@@ -26,12 +26,10 @@ public sealed partial class AgentService
     private readonly LinkService _links;
     private readonly TrustService _trust;
 
-    private readonly BinaryBindingService _bindings;
     private readonly NodeOwnerService _owners;
 
-    public AgentService(BinaryBindingService bindings, NodeOwnerService owners, IAgentStore agents, AgentRuntime runtime, IAgentGateway gateway, TaskPlanner planner, ISettingsStore settings, NodeInfoService nodes, LinkService links, TrustService trust)
+    public AgentService(NodeOwnerService owners, IAgentStore agents, AgentRuntime runtime, IAgentGateway gateway, TaskPlanner planner, ISettingsStore settings, NodeInfoService nodes, LinkService links, TrustService trust)
     {
-        _bindings = bindings;
         _owners = owners;
         _trust = trust;
         _links = links;
@@ -152,7 +150,6 @@ public sealed partial class AgentService
         await _gateway.DisconnectAsync(id, "deleted", ct);
         await _agents.DeleteAsync(id, ct);
         await _nodes.ForgetAsync(id.Value, ct);
-        await _bindings.ForgetAsync(id.Value, ct);
         _owners.Forget(id.Value);
         await _links.ForgetNodeAsync(id.Value, ct);
         _runtime.Remove(id);