// 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 and optional engine-scoped [Backend] registrations, and
// call [Engine.Prepare] or [Engine.Run]. Concrete registries, repositories,
// validators, and the built-in OpenAI-compatible client remain internal
// implementation details.
//
// # Concurrency and ownership
//
// An Engine supports concurrent Prepare and Run calls. Engine-local backend
// policies bound admitted Run calls and model generations where configured,
// while different backend pools and unlimited backends continue independently.
// An injected [LLMClient] or [ArtifactReader] can therefore still receive
// concurrent calls and must be safe for that use.
//
// NewEngine copies in-memory profiles and backend definitions. 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.
//
// # Security and sensitive data
//
// The default artifact reader treats [File] paths as caller-selected operating
// system paths. It does not restrict them to an application root or impose an
// inbound request-size policy. Promptkit is not an inbound request or
// untrusted-input security boundary. Applications must validate and restrict
// untrusted input before constructing a request, or install an [ArtifactReader]
// that enforces their filesystem, authorization, and size policies.
//
// Rendered messages, input and output [Artifact] bodies, [RunResult.RawOutput],
// and [ValidationResult.Errors] may contain sensitive data. Credential
// exclusion and redaction do not sanitize those values. Applications and
// injected collaborators are responsible for access control, retention,
// logging, and secret handling appropriate to their data.
//
// # 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], [Backend], [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