feat: ReportPackages wire types with hash short-circuit

The pb packages are hand-written, not protoc-generated, and the wire
codec is JSON (encoding.RegisterCodec(JSONCodec{})). Field numbers in
the .proto are documentation; JSON field names are the contract. Both pb
packages edited by hand to match.

SyncResponse.collect_packages is omitempty and absent decodes as false,
so an older server leaves agents collecting nothing rather than
collecting without a licence.

proto/vantage/v1/vantage.proto

@@ -9,6 +9,7 @@ service Vantage {
   rpc SyncKeys(SyncRequest)                    returns (SyncResponse);
   rpc UploadGeneratedKey(UploadKeyRequest)      returns (UploadKeyResponse);
   rpc ReportUpdates(ReportUpdatesRequest)       returns (ReportUpdatesResponse);
+  rpc ReportPackages(ReportPackagesRequest)     returns (ReportPackagesResponse);
   rpc ReportInventory(InventoryReport)        returns (InventoryReportResponse);
   rpc SyncMonitors(SyncMonitorsRequest)         returns (SyncMonitorsResponse);
   rpc ReportChecks(ReportChecksRequest)         returns (ReportChecksResponse);
@@ -37,6 +38,51 @@ message SyncRequest {
 
 message SyncResponse {
   repeated string public_keys = 1;
+
+  // collect_packages tells the agent whether this instance's licence grants
+  // vulnerability scanning. False means do not collect at all: no gRPC body,
+  // no document, no storage. The server re-checks on ReportPackages — this
+  // flag is the optimisation, the server check is the boundary.
+  //
+  // Absent reads as false, which is the safe direction: an old server that
+  // does not send it leaves agents collecting nothing.
+  bool collect_packages = 2;
+}
+
+// ReportPackages carries a server's installed package set.
+//
+// The agent calls twice at most. The first call sends only the hash; if the
+// server already holds that hash it answers need_full = false and the ~150KB
+// body is never sent. A machine's package set changes rarely, so almost every
+// hour costs one small message.
+message ReportPackagesRequest {
+  string server_id   = 1;
+  string agent_token = 2;
+  string hash        = 3;
+  OSRelease os       = 4;
+  repeated InstalledPackage packages = 5;  // empty on the offer call
+}
+
+message ReportPackagesResponse {
+  bool need_full = 1;
+}
+
+message OSRelease {
+  string family     = 1;
+  // version_id is not optional: Ubuntu 22.04 and 24.04 publish different fixed
+  // versions for the same CVE, so a scan without it is guesswork.
+  string version_id = 2;
+  string arch       = 3;
+}
+
+message InstalledPackage {
+  string name    = 1;
+  string version = 2;
+  int32  epoch   = 3;
+  string arch    = 4;
+  // source_name is what the Debian and Ubuntu feeds are keyed on: one advisory
+  // against "openssl" covers libssl3, openssl and libssl-dev.
+  string source_name = 5;
 }
 
 message UploadKeyRequest {