// Package promptkit provides an embeddable engine for preparing and executing
// prompt-defined LLM workflows.
//
// Applications construct an [Engine] with [NewEngine], select filesystem or
// in-memory sources with options, and call [Engine.Prepare] or [Engine.Run].
// Concrete repositories, validators, and the built-in OpenAI-compatible client
// remain internal implementation details.
//
// # Concurrency and ownership
//
// An Engine supports concurrent Prepare and Run calls. An injected [LLMClient]
// or [ArtifactReader] can therefore receive concurrent calls and must be safe
// for that use.
//
// NewEngine copies in-memory profiles. Prepare and Run copy request maps,
// slices, pointer values, and JSON-compatible extra parameters before using
// them. Returned values and values passed to extension interfaces are likewise
// isolated from engine state. Callers own those copies and may mutate them
// after the call that supplied or returned them.
//
// # JSON
//
// Stable JSON representations are provided for [PreparedRun], [RunResult],
// [Artifact], [ExecutionTarget], [OutputContract], [ValidationResult],
// [TokenUsage], [RenderedPrompt], [RenderedMessage], [CacheControl],
// [StructuredOutputSpec], [StructuredOutputJSONSpec], [GenerateRequest],
// [GenerateResponse], [ExecutionTargetPresence], and the string value types
// used by those values.
//
// Construction values, including [Config], [RunRequest], [ArtifactRef],
// [ExecutionTargetOverride], [Profile], and
// [OpenAICompatibleProfileConfig], do not have stable JSON representations.
// Direct API keys are nevertheless excluded from JSON for every public value.
//
// JSON timestamps use time.Time's RFC 3339 encoding and are omitted when zero.
// PreparedRun and RunResult durations are encoded as integer milliseconds in
// duration_ms and omitted when zero. Run IDs and all exposed hashes are opaque:
// their spelling, length, character set, and algorithm are not API contracts.
package promptkit