feat: add MCP server instructions for flake.lock indexing

Provides guidance to AI assistants on how to index the nixpkgs
revision from a project's flake.lock file, ensuring option
documentation matches the project's actual nixpkgs version.

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

internal/mcp/server.go

@@ -149,6 +149,15 @@ func (s *Server) handleInitialize(req *Request) *Response {
 			Name:    "nixos-options",
 			Version: "0.1.0",
 		},
+		Instructions: `NixOS Options MCP Server - Search and query NixOS configuration options.
+
+If the current project contains a flake.lock file, you can index the exact nixpkgs revision used by the project:
+1. Read the flake.lock file to find the nixpkgs "rev" field
+2. Call index_revision with that git hash to index options for that specific version
+
+Example: If flake.lock contains "rev": "abc123...", call index_revision with revision "abc123...".
+
+This ensures option documentation matches the nixpkgs version the project actually uses.`,
 	}
 
 	return &Response{