feat: add lab-monitoring MCP server for Prometheus and Alertmanager

New MCP server that queries live Prometheus and Alertmanager HTTP APIs
with 8 tools: list_alerts, get_alert, search_metrics, get_metric_metadata,
query (PromQL), list_targets, list_silences, and create_silence.

Extends the MCP core with ModeCustom and NewGenericServer for servers
that don't require a database. Includes CLI with direct commands
(alerts, query, targets, metrics), NixOS module, and comprehensive
httptest-based tests.

Bumps existing binaries to 0.2.1 due to shared internal/mcp change.

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

internal/mcp/server.go

@@ -18,6 +18,8 @@ const (
 	ModeOptions ServerMode = "options"
 	// ModePackages exposes only package-related tools.
 	ModePackages ServerMode = "packages"
+	// ModeCustom exposes externally registered tools (no database required).
+	ModeCustom ServerMode = "custom"
 )
 
 // ServerConfig contains configuration for the MCP server.
@@ -40,7 +42,7 @@ type ServerConfig struct {
 func DefaultNixOSConfig() ServerConfig {
 	return ServerConfig{
 		Name:           "nixos-options",
-		Version:        "0.2.0",
+		Version:        "0.2.1",
 		DefaultChannel: "nixos-stable",
 		SourceName:     "nixpkgs",
 		Mode:           ModeOptions,
@@ -60,7 +62,7 @@ This ensures option documentation matches the nixpkgs version the project actual
 func DefaultNixpkgsPackagesConfig() ServerConfig {
 	return ServerConfig{
 		Name:           "nixpkgs-packages",
-		Version:        "0.2.0",
+		Version:        "0.2.1",
 		DefaultChannel: "nixos-stable",
 		SourceName:     "nixpkgs",
 		Mode:           ModePackages,
@@ -74,11 +76,29 @@ This ensures package information matches the nixpkgs version the project actuall
 	}
 }
 
+// DefaultMonitoringConfig returns the default configuration for the lab monitoring server.
+func DefaultMonitoringConfig() ServerConfig {
+	return ServerConfig{
+		Name:    "lab-monitoring",
+		Version: "0.1.0",
+		Mode:    ModeCustom,
+		Instructions: `Lab Monitoring MCP Server - Query Prometheus metrics and Alertmanager alerts.
+
+Tools for querying your monitoring stack:
+- Search and query Prometheus metrics with PromQL
+- List and inspect alerts from Alertmanager
+- View scrape target health status
+- Manage alert silences
+
+All queries are executed against live Prometheus and Alertmanager HTTP APIs.`,
+	}
+}
+
 // DefaultHomeManagerConfig returns the default configuration for Home Manager options server.
 func DefaultHomeManagerConfig() ServerConfig {
 	return ServerConfig{
 		Name:           "hm-options",
-		Version:        "0.2.0",
+		Version:        "0.2.1",
 		DefaultChannel: "hm-stable",
 		SourceName:     "home-manager",
 		Mode:           ModeOptions,
@@ -99,6 +119,7 @@ type Server struct {
 	store       database.Store
 	config      ServerConfig
 	tools       map[string]ToolHandler
+	toolDefs    []Tool
 	initialized bool
 	logger      *log.Logger
 }
@@ -106,7 +127,7 @@ type Server struct {
 // ToolHandler is a function that handles a tool call.
 type ToolHandler func(ctx context.Context, args map[string]interface{}) (CallToolResult, error)
 
-// NewServer creates a new MCP server with the given configuration.
+// NewServer creates a new MCP server with a database store.
 func NewServer(store database.Store, logger *log.Logger, config ServerConfig) *Server {
 	if logger == nil {
 		logger = log.New(io.Discard, "", 0)
@@ -121,6 +142,25 @@ func NewServer(store database.Store, logger *log.Logger, config ServerConfig) *S
 	return s
 }
 
+// NewGenericServer creates a new MCP server without a database store.
+// Use RegisterTool to add tools externally.
+func NewGenericServer(logger *log.Logger, config ServerConfig) *Server {
+	if logger == nil {
+		logger = log.New(io.Discard, "", 0)
+	}
+	return &Server{
+		config: config,
+		tools:  make(map[string]ToolHandler),
+		logger: logger,
+	}
+}
+
+// RegisterTool registers an externally defined tool with its handler.
+func (s *Server) RegisterTool(tool Tool, handler ToolHandler) {
+	s.toolDefs = append(s.toolDefs, tool)
+	s.tools[tool.Name] = handler
+}
+
 // registerTools registers all available tools.
 func (s *Server) registerTools() {
 	// Tools will be implemented in handlers.go
@@ -237,6 +277,11 @@ func (s *Server) handleToolsList(req *Request) *Response {
 
 // getToolDefinitions returns the tool definitions.
 func (s *Server) getToolDefinitions() []Tool {
+	// For custom mode, return externally registered tools
+	if s.config.Mode == ModeCustom {
+		return s.toolDefs
+	}
+
 	// For packages mode, return package tools
 	if s.config.Mode == ModePackages {
 		return s.getPackageToolDefinitions()