feat: include active alert count in MCP server instructions

Add InstructionsFunc callback to ServerConfig, called during each
initialize handshake to generate dynamic instructions. The lab-monitoring
server uses this to query Alertmanager and include a count of active
non-silenced alerts, so the LLM can proactively inform the user.

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

internal/mcp/server.go

@@ -30,6 +30,9 @@ type ServerConfig struct {
 	Version string
 	// Instructions are the server instructions sent to clients.
 	Instructions string
+	// InstructionsFunc, if set, is called during initialization to generate
+	// dynamic instructions. Its return value is appended to Instructions.
+	InstructionsFunc func() string
 	// DefaultChannel is the default channel to use when no revision is specified.
 	DefaultChannel string
 	// SourceName is the name of the source repository (e.g., "nixpkgs", "home-manager").
@@ -244,6 +247,13 @@ func (s *Server) handleInitialize(req *Request) *Response {
 	s.logger.Printf("Client: %s %s, protocol: %s",
 		params.ClientInfo.Name, params.ClientInfo.Version, params.ProtocolVersion)
 
+	instructions := s.config.Instructions
+	if s.config.InstructionsFunc != nil {
+		if extra := s.config.InstructionsFunc(); extra != "" {
+			instructions += "\n\n" + extra
+		}
+	}
+
 	result := InitializeResult{
 		ProtocolVersion: ProtocolVersion,
 		Capabilities: Capabilities{
@@ -255,7 +265,7 @@ func (s *Server) handleInitialize(req *Request) *Response {
 			Name:    s.config.Name,
 			Version: s.config.Version,
 		},
-		Instructions: s.config.Instructions,
+		Instructions: instructions,
 	}
 
 	return &Response{