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/database/interface.go

@@ -8,12 +8,13 @@ import (
 
 // Revision represents an indexed nixpkgs revision.
 type Revision struct {
-	ID          int64
-	GitHash     string
-	ChannelName string
-	CommitDate  time.Time
-	IndexedAt   time.Time
-	OptionCount int
+	ID           int64
+	GitHash      string
+	ChannelName  string
+	CommitDate   time.Time
+	IndexedAt    time.Time
+	OptionCount  int
+	PackageCount int
 }
 
 // Option represents a NixOS configuration option.
@@ -48,6 +49,24 @@ type File struct {
 	LineCount  int
 }
 
+// Package represents a Nix package from nixpkgs.
+type Package struct {
+	ID              int64
+	RevisionID      int64
+	AttrPath        string // e.g., "python312Packages.requests"
+	Pname           string // Package name
+	Version         string
+	Description     string
+	LongDescription string
+	Homepage        string
+	License         string // JSON array
+	Platforms       string // JSON array
+	Maintainers     string // JSON array
+	Broken          bool
+	Unfree          bool
+	Insecure        bool
+}
+
 // DeclarationWithMetadata includes declaration info plus file metadata.
 type DeclarationWithMetadata struct {
 	Declaration
@@ -79,6 +98,15 @@ type SearchFilters struct {
 	Offset     int
 }
 
+// PackageSearchFilters contains optional filters for package search.
+type PackageSearchFilters struct {
+	Broken   *bool
+	Unfree   *bool
+	Insecure *bool
+	Limit    int
+	Offset   int
+}
+
 // Store defines the interface for database operations.
 type Store interface {
 	// Schema operations
@@ -111,4 +139,11 @@ type Store interface {
 	CreateFilesBatch(ctx context.Context, files []*File) error
 	GetFile(ctx context.Context, revisionID int64, path string) (*File, error)
 	GetFileWithRange(ctx context.Context, revisionID int64, path string, r FileRange) (*FileResult, error)
+
+	// Package operations
+	CreatePackage(ctx context.Context, pkg *Package) error
+	CreatePackagesBatch(ctx context.Context, pkgs []*Package) error
+	GetPackage(ctx context.Context, revisionID int64, attrPath string) (*Package, error)
+	SearchPackages(ctx context.Context, revisionID int64, query string, filters PackageSearchFilters) ([]*Package, error)
+	UpdateRevisionPackageCount(ctx context.Context, id int64, count int) error
 }