feat: add nixpkgs-search binary with package search support

Add a new nixpkgs-search CLI that combines NixOS options search with
Nix package search functionality. This provides two MCP servers from
a single binary:
- `nixpkgs-search options serve` for NixOS options
- `nixpkgs-search packages serve` for Nix packages

Key changes:
- Add packages table to database schema (version 3)
- Add Package type and search methods to database layer
- Create internal/packages/ with indexer and parser for nix-env JSON
- Add MCP server mode (options/packages) with separate tool sets
- Add package handlers: search_packages, get_package
- Create cmd/nixpkgs-search with combined indexing support
- Update flake.nix with nixpkgs-search package (now default)
- Bump version to 0.2.0

The index command can index both options and packages together, or
use --no-packages/--no-options flags for partial indexing.

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

internal/packages/parser.go

@@ -0,0 +1,199 @@
+package packages
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"strings"
+)
+
+// ParsePackages reads and parses a nix-env JSON output file.
+func ParsePackages(r io.Reader) (map[string]*ParsedPackage, error) {
+	var raw PackagesFile
+	if err := json.NewDecoder(r).Decode(&raw); err != nil {
+		return nil, fmt.Errorf("failed to decode packages JSON: %w", err)
+	}
+
+	packages := make(map[string]*ParsedPackage, len(raw))
+	for attrPath, pkg := range raw {
+		parsed := &ParsedPackage{
+			AttrPath:        attrPath,
+			Pname:           pkg.Pname,
+			Version:         pkg.Version,
+			Description:     pkg.Meta.Description,
+			LongDescription: pkg.Meta.LongDescription,
+			Homepage:        normalizeHomepage(pkg.Meta.Homepage),
+			License:         normalizeLicense(pkg.Meta.License),
+			Platforms:       normalizePlatforms(pkg.Meta.Platforms),
+			Maintainers:     normalizeMaintainers(pkg.Meta.Maintainers),
+			Broken:          pkg.Meta.Broken,
+			Unfree:          pkg.Meta.Unfree,
+			Insecure:        pkg.Meta.Insecure,
+		}
+		packages[attrPath] = parsed
+	}
+
+	return packages, nil
+}
+
+// normalizeHomepage converts homepage to a string.
+func normalizeHomepage(v interface{}) string {
+	if v == nil {
+		return ""
+	}
+	switch hp := v.(type) {
+	case string:
+		return hp
+	case []interface{}:
+		if len(hp) > 0 {
+			if s, ok := hp[0].(string); ok {
+				return s
+			}
+		}
+	}
+	return ""
+}
+
+// normalizeLicense converts license to a JSON array string.
+func normalizeLicense(v interface{}) string {
+	if v == nil {
+		return "[]"
+	}
+
+	licenses := make([]string, 0)
+
+	switch l := v.(type) {
+	case string:
+		licenses = append(licenses, l)
+	case map[string]interface{}:
+		// Single license object
+		if spdxID, ok := l["spdxId"].(string); ok {
+			licenses = append(licenses, spdxID)
+		} else if fullName, ok := l["fullName"].(string); ok {
+			licenses = append(licenses, fullName)
+		} else if shortName, ok := l["shortName"].(string); ok {
+			licenses = append(licenses, shortName)
+		}
+	case []interface{}:
+		for _, item := range l {
+			switch li := item.(type) {
+			case string:
+				licenses = append(licenses, li)
+			case map[string]interface{}:
+				if spdxID, ok := li["spdxId"].(string); ok {
+					licenses = append(licenses, spdxID)
+				} else if fullName, ok := li["fullName"].(string); ok {
+					licenses = append(licenses, fullName)
+				} else if shortName, ok := li["shortName"].(string); ok {
+					licenses = append(licenses, shortName)
+				}
+			}
+		}
+	}
+
+	data, _ := json.Marshal(licenses)
+	return string(data)
+}
+
+// normalizePlatforms converts platforms to a JSON array string.
+func normalizePlatforms(v []interface{}) string {
+	if v == nil {
+		return "[]"
+	}
+
+	platforms := make([]string, 0, len(v))
+	for _, p := range v {
+		switch pv := p.(type) {
+		case string:
+			platforms = append(platforms, pv)
+			// Skip complex platform specs (objects)
+		}
+	}
+
+	data, _ := json.Marshal(platforms)
+	return string(data)
+}
+
+// normalizeMaintainers converts maintainers to a JSON array string.
+func normalizeMaintainers(maintainers []Maintainer) string {
+	if len(maintainers) == 0 {
+		return "[]"
+	}
+
+	names := make([]string, 0, len(maintainers))
+	for _, m := range maintainers {
+		name := m.Name
+		if name == "" && m.Github != "" {
+			name = "@" + m.Github
+		}
+		if name != "" {
+			names = append(names, name)
+		}
+	}
+
+	data, _ := json.Marshal(names)
+	return string(data)
+}
+
+// ParsePackagesStream parses packages from a reader using streaming to reduce memory usage.
+// It yields parsed packages through a callback function.
+func ParsePackagesStream(r io.Reader, callback func(*ParsedPackage) error) (int, error) {
+	dec := json.NewDecoder(r)
+
+	// Read the opening brace
+	t, err := dec.Token()
+	if err != nil {
+		return 0, fmt.Errorf("failed to read opening token: %w", err)
+	}
+	if delim, ok := t.(json.Delim); !ok || delim != '{' {
+		return 0, fmt.Errorf("expected opening brace, got %v", t)
+	}
+
+	count := 0
+	for dec.More() {
+		// Read the key (attr path)
+		t, err := dec.Token()
+		if err != nil {
+			return count, fmt.Errorf("failed to read attr path: %w", err)
+		}
+		attrPath, ok := t.(string)
+		if !ok {
+			return count, fmt.Errorf("expected string key, got %T", t)
+		}
+
+		// Read the value (package)
+		var pkg RawPackage
+		if err := dec.Decode(&pkg); err != nil {
+			// Skip malformed packages
+			continue
+		}
+
+		parsed := &ParsedPackage{
+			AttrPath:        attrPath,
+			Pname:           pkg.Pname,
+			Version:         pkg.Version,
+			Description:     pkg.Meta.Description,
+			LongDescription: pkg.Meta.LongDescription,
+			Homepage:        normalizeHomepage(pkg.Meta.Homepage),
+			License:         normalizeLicense(pkg.Meta.License),
+			Platforms:       normalizePlatforms(pkg.Meta.Platforms),
+			Maintainers:     normalizeMaintainers(pkg.Meta.Maintainers),
+			Broken:          pkg.Meta.Broken,
+			Unfree:          pkg.Meta.Unfree,
+			Insecure:        pkg.Meta.Insecure,
+		}
+
+		if err := callback(parsed); err != nil {
+			return count, fmt.Errorf("callback error for %s: %w", attrPath, err)
+		}
+		count++
+	}
+
+	return count, nil
+}
+
+// SplitAttrPath splits an attribute path into its components.
+// For example, "python312Packages.requests" returns ["python312Packages", "requests"].
+func SplitAttrPath(attrPath string) []string {
+	return strings.Split(attrPath, ".")
+}