Add CapabilitiesJson field to DeclareRequest

This allows runners to report their capabilities (OS, arch, docker,
tools, etc.) as a JSON string when declaring themselves to Gitea.

This enables AI-friendly workflow generation by providing structured
capability information that AI tools can query before writing workflows.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

runner/v1/messages.pb.go

@@ -282,8 +282,9 @@ type DeclareRequest struct {
 	sizeCache     protoimpl.SizeCache
 	unknownFields protoimpl.UnknownFields
 
-	Version string   `protobuf:"bytes,1,opt,name=version,proto3" json:"version,omitempty"`
-	Labels  []string `protobuf:"bytes,2,rep,name=labels,proto3" json:"labels,omitempty"`
+	Version          string   `protobuf:"bytes,1,opt,name=version,proto3" json:"version,omitempty"`
+	Labels           []string `protobuf:"bytes,2,rep,name=labels,proto3" json:"labels,omitempty"`
+	CapabilitiesJson string   `protobuf:"bytes,3,opt,name=capabilities_json,json=capabilitiesJson,proto3" json:"capabilities_json,omitempty"`
 }
 
 func (x *DeclareRequest) Reset() {
@@ -332,6 +333,13 @@ func (x *DeclareRequest) GetLabels() []string {
 	return nil
 }
 
+func (x *DeclareRequest) GetCapabilitiesJson() string {
+	if x != nil {
+		return x.CapabilitiesJson
+	}
+	return ""
+}
+
 type DeclareResponse struct {
 	state         protoimpl.MessageState
 	sizeCache     protoimpl.SizeCache