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/database/interface.go

@@ -0,0 +1,88 @@
+// Package database provides database abstraction for storing NixOS options.
+package database
+
+import (
+	"context"
+	"time"
+)
+
+// Revision represents an indexed nixpkgs revision.
+type Revision struct {
+	ID          int64
+	GitHash     string
+	ChannelName string
+	CommitDate  time.Time
+	IndexedAt   time.Time
+	OptionCount int
+}
+
+// Option represents a NixOS configuration option.
+type Option struct {
+	ID           int64
+	RevisionID   int64
+	Name         string
+	ParentPath   string
+	Type         string
+	DefaultValue string // JSON text
+	Example      string // JSON text
+	Description  string
+	ReadOnly     bool
+}
+
+// Declaration represents a file where an option is declared.
+type Declaration struct {
+	ID       int64
+	OptionID int64
+	FilePath string
+	Line     int
+}
+
+// File represents a cached file from nixpkgs.
+type File struct {
+	ID         int64
+	RevisionID int64
+	FilePath   string
+	Extension  string
+	Content    string
+}
+
+// SearchFilters contains optional filters for option search.
+type SearchFilters struct {
+	Type       string
+	Namespace  string
+	HasDefault *bool
+	Limit      int
+	Offset     int
+}
+
+// Store defines the interface for database operations.
+type Store interface {
+	// Schema operations
+	Initialize(ctx context.Context) error
+	Close() error
+
+	// Revision operations
+	CreateRevision(ctx context.Context, rev *Revision) error
+	GetRevision(ctx context.Context, gitHash string) (*Revision, error)
+	GetRevisionByChannel(ctx context.Context, channel string) (*Revision, error)
+	ListRevisions(ctx context.Context) ([]*Revision, error)
+	DeleteRevision(ctx context.Context, id int64) error
+	UpdateRevisionOptionCount(ctx context.Context, id int64, count int) error
+
+	// Option operations
+	CreateOption(ctx context.Context, opt *Option) error
+	CreateOptionsBatch(ctx context.Context, opts []*Option) error
+	GetOption(ctx context.Context, revisionID int64, name string) (*Option, error)
+	GetChildren(ctx context.Context, revisionID int64, parentPath string) ([]*Option, error)
+	SearchOptions(ctx context.Context, revisionID int64, query string, filters SearchFilters) ([]*Option, error)
+
+	// Declaration operations
+	CreateDeclaration(ctx context.Context, decl *Declaration) error
+	CreateDeclarationsBatch(ctx context.Context, decls []*Declaration) error
+	GetDeclarations(ctx context.Context, optionID int64) ([]*Declaration, error)
+
+	// File operations
+	CreateFile(ctx context.Context, file *File) error
+	CreateFilesBatch(ctx context.Context, files []*File) error
+	GetFile(ctx context.Context, revisionID int64, path string) (*File, error)
+}