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/schema.go

@@ -2,7 +2,7 @@ package database
 
 // SchemaVersion is the current database schema version.
 // When this changes, the database will be dropped and recreated.
-const SchemaVersion = 2
+const SchemaVersion = 3
 
 // Common SQL statements shared between implementations.
 const (
@@ -20,7 +20,8 @@ const (
 			channel_name TEXT,
 			commit_date TIMESTAMP,
 			indexed_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
-			option_count INTEGER NOT NULL DEFAULT 0
+			option_count INTEGER NOT NULL DEFAULT 0,
+			package_count INTEGER NOT NULL DEFAULT 0
 		)`
 
 	// OptionsTable creates the options table.
@@ -57,6 +58,25 @@ const (
 			byte_size INTEGER NOT NULL DEFAULT 0,
 			line_count INTEGER NOT NULL DEFAULT 0
 		)`
+
+	// PackagesTable creates the packages table.
+	PackagesTable = `
+		CREATE TABLE IF NOT EXISTS packages (
+			id INTEGER PRIMARY KEY,
+			revision_id INTEGER NOT NULL REFERENCES revisions(id) ON DELETE CASCADE,
+			attr_path TEXT NOT NULL,
+			pname TEXT NOT NULL,
+			version TEXT,
+			description TEXT,
+			long_description TEXT,
+			homepage TEXT,
+			license TEXT,
+			platforms TEXT,
+			maintainers TEXT,
+			broken BOOLEAN NOT NULL DEFAULT FALSE,
+			unfree BOOLEAN NOT NULL DEFAULT FALSE,
+			insecure BOOLEAN NOT NULL DEFAULT FALSE
+		)`
 )
 
 // Index creation statements.
@@ -80,6 +100,16 @@ const (
 	IndexDeclarationsOption = `
 		CREATE INDEX IF NOT EXISTS idx_declarations_option
 		ON declarations(option_id)`
+
+	// IndexPackagesRevisionAttr creates an index on packages(revision_id, attr_path).
+	IndexPackagesRevisionAttr = `
+		CREATE UNIQUE INDEX IF NOT EXISTS idx_packages_revision_attr
+		ON packages(revision_id, attr_path)`
+
+	// IndexPackagesRevisionPname creates an index on packages(revision_id, pname).
+	IndexPackagesRevisionPname = `
+		CREATE INDEX IF NOT EXISTS idx_packages_revision_pname
+		ON packages(revision_id, pname)`
 )
 
 // Drop statements for schema recreation.
@@ -87,6 +117,7 @@ const (
 	DropSchemaInfo   = `DROP TABLE IF EXISTS schema_info`
 	DropDeclarations = `DROP TABLE IF EXISTS declarations`
 	DropOptions      = `DROP TABLE IF EXISTS options`
+	DropPackages     = `DROP TABLE IF EXISTS packages`
 	DropFiles        = `DROP TABLE IF EXISTS files`
 	DropRevisions    = `DROP TABLE IF EXISTS revisions`
 )