Add HTTP whole-run deadlines and context outcomes

docs/api.md

@@ -51,6 +51,7 @@ copyable shape. At the HTTP adapter boundary, the smallest valid shape is:
 | `model` | no | Runtime model-override object. |
 | `include_raw_output` | no | Include `raw_model_output` when true. |
 | `transient_retries` | no | Nonnegative additional transient attempt count for this run. Omitted or `null` inherits the configured engine value and then Promptkit's default; `0` disables additional attempts. |
+| `run_timeout_seconds` | no | Optional signed 64-bit whole-second limit for engine execution. Omitted, `null`, or `0` adds no deadline; positive values shorten the caller context deadline. |
 
 An input reference has a required `type` of `file` or `inline`. A `file`
 reference requires `uri`; an `inline` reference requires `body`.
@@ -98,6 +99,12 @@ returns `400 invalid_json`, while a negative integer returns
 `400 invalid_request` before execution. A request override does not change the
 shared engine setting used by other requests. Promptkit owns eligible statuses,
 retry waits, and the separate transient budget for each repair generation.
+`run_timeout_seconds` must be a JSON int64. A wrong type or int64 overflow
+returns `400 invalid_json`; a negative value or one whose conversion to a Go
+duration would overflow returns `400 invalid_request`. The deadline starts
+immediately before engine execution and excludes body decoding and response
+writing. An earlier caller deadline or cancellation still wins. Synchronous
+work is not guaranteed to stop immediately.
 
 ### Success Response
 
@@ -157,6 +164,7 @@ Messages are concise and do not expose wrapped internal causes.
 | `404` | `prompt_not_found` | Prompt ID or version does not exist. |
 | `404` | `profile_not_found` | Profile ID does not exist. |
 | `405` | `method_not_allowed` | The route does not accept the method. |
+| `408` | `request_canceled` | The caller context was canceled during a failed run. |
 | `413` | `request_too_large` | Encoded request exceeds its limit. |
 | `413` | `artifact_too_large` | File input exceeds its limit. |
 | `413` | `response_too_large` | Encoded response exceeds its limit. |
@@ -164,6 +172,12 @@ Messages are concise and do not expose wrapped internal causes.
 | `500` | `internal_error` | Unclassified server failure. |
 | `502` | `llm_failed` | Outbound model request failed. |
 | `503` | `capacity_exceeded` | The selected model backend has no admission capacity. No retry timing is supplied. |
+| `504` | `deadline_exceeded` | The execution context's deadline expired during a failed run. |
+
+These context outcomes require the execution context to have ended. A provider
+attempt timeout while it remains live retains `502 llm_failed`. A successful
+run remains successful even if cancellation races with response formatting;
+delivery after a client disconnect cannot be guaranteed.
 
 ## Retry And Idempotency