feat: project structure and nix build setup

- Add CLI entry point with urfave/cli/v2 (serve, index, list, search commands)
- Add database interface and implementations for PostgreSQL and SQLite
- Add schema versioning with automatic recreation on version mismatch
- Add MCP protocol types and server scaffold
- Add NixOS option types
- Configure flake.nix with devShell and buildGoModule package

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

cmd/nixos-options/main.go

@@ -0,0 +1,99 @@
+package main
+
+import (
+	"fmt"
+	"log"
+	"os"
+
+	"github.com/urfave/cli/v2"
+)
+
+func main() {
+	app := &cli.App{
+		Name:  "nixos-options",
+		Usage: "MCP server for NixOS options search and query",
+		Commands: []*cli.Command{
+			{
+				Name:  "serve",
+				Usage: "Run MCP server (stdio)",
+				Flags: []cli.Flag{
+					&cli.StringFlag{
+						Name:    "database",
+						Aliases: []string{"d"},
+						Usage:   "Database connection string (postgres://... or sqlite://...)",
+						EnvVars: []string{"NIXOS_OPTIONS_DATABASE"},
+					},
+				},
+				Action: func(c *cli.Context) error {
+					fmt.Println("MCP server not yet implemented")
+					return nil
+				},
+			},
+			{
+				Name:      "index",
+				Usage:     "Index a nixpkgs revision",
+				ArgsUsage: "<revision>",
+				Flags: []cli.Flag{
+					&cli.StringFlag{
+						Name:    "database",
+						Aliases: []string{"d"},
+						Usage:   "Database connection string",
+						EnvVars: []string{"NIXOS_OPTIONS_DATABASE"},
+					},
+				},
+				Action: func(c *cli.Context) error {
+					if c.NArg() < 1 {
+						return fmt.Errorf("revision argument required")
+					}
+					fmt.Printf("Indexing revision: %s\n", c.Args().First())
+					return nil
+				},
+			},
+			{
+				Name:  "list",
+				Usage: "List indexed revisions",
+				Flags: []cli.Flag{
+					&cli.StringFlag{
+						Name:    "database",
+						Aliases: []string{"d"},
+						Usage:   "Database connection string",
+						EnvVars: []string{"NIXOS_OPTIONS_DATABASE"},
+					},
+				},
+				Action: func(c *cli.Context) error {
+					fmt.Println("List revisions not yet implemented")
+					return nil
+				},
+			},
+			{
+				Name:      "search",
+				Usage:     "Search for options",
+				ArgsUsage: "<query>",
+				Flags: []cli.Flag{
+					&cli.StringFlag{
+						Name:    "database",
+						Aliases: []string{"d"},
+						Usage:   "Database connection string",
+						EnvVars: []string{"NIXOS_OPTIONS_DATABASE"},
+					},
+					&cli.StringFlag{
+						Name:    "revision",
+						Aliases: []string{"r"},
+						Usage:   "Revision to search (default: nixos-stable)",
+					},
+				},
+				Action: func(c *cli.Context) error {
+					if c.NArg() < 1 {
+						return fmt.Errorf("query argument required")
+					}
+					fmt.Printf("Searching for: %s\n", c.Args().First())
+					return nil
+				},
+			},
+		},
+	}
+
+	if err := app.Run(os.Args); err != nil {
+		log.Fatal(err)
+	}
+}