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>

internal/nixos/types.go

@@ -0,0 +1,45 @@
+// Package nixos contains types and logic specific to NixOS options.
+package nixos
+
+// RawOption represents an option as parsed from options.json.
+// The structure matches the output of `nix-build '<nixpkgs/nixos/release.nix>' -A options`.
+type RawOption struct {
+	Declarations []string     `json:"declarations"`
+	Default      *OptionValue `json:"default,omitempty"`
+	Description  string       `json:"description"`
+	Example      *OptionValue `json:"example,omitempty"`
+	ReadOnly     bool         `json:"readOnly"`
+	Type         string       `json:"type"`
+	Loc          []string     `json:"loc,omitempty"`
+}
+
+// OptionValue wraps a value that may be a literal or a Nix expression.
+type OptionValue struct {
+	// Text is the raw JSON representation of the value
+	Text string
+}
+
+// OptionsFile represents the top-level structure of options.json.
+// It's a map from option name to option definition.
+type OptionsFile map[string]RawOption
+
+// AllowedExtensions is the default set of file extensions to index.
+var AllowedExtensions = map[string]bool{
+	".nix":  true,
+	".json": true,
+	".md":   true,
+	".txt":  true,
+	".toml": true,
+	".yaml": true,
+	".yml":  true,
+}
+
+// ChannelAliases maps friendly channel names to their git branch/ref patterns.
+var ChannelAliases = map[string]string{
+	"nixos-unstable": "nixos-unstable",
+	"nixos-stable":   "nixos-24.11", // Update this as new stable releases come out
+	"nixos-24.11":    "nixos-24.11",
+	"nixos-24.05":    "nixos-24.05",
+	"nixos-23.11":    "nixos-23.11",
+	"nixos-23.05":    "nixos-23.05",
+}