Expose HTTP transient retry control and preserve shared capacity

docs/api.md

@@ -50,6 +50,7 @@ copyable shape. At the HTTP adapter boundary, the smallest valid shape is:
 | `vars` | no | Object mapping template-variable names to strings. |
 | `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. |
 
 An input reference has a required `type` of `file` or `inline`. A `file`
 reference requires `uri`; an `inline` reference requires `body`.
@@ -92,6 +93,11 @@ Request decoding rejects malformed JSON, unknown fields at every request level,
 and trailing JSON tokens with `400 invalid_json`. A blank `prompt_id` returns
 `400 invalid_request`. Omitted or empty `inputs` are passed to Promptkit, which
 reports any definition-required or template-referenced inputs.
+`transient_retries` must be a JSON integer; a wrong type or noninteger number
+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.
 
 ### Success Response