Document runtime provider parameters

docs/internal/runner.md

@@ -103,6 +103,7 @@ Validation content failures are not run errors:
    - built-in execution defaults
    - selected profile values
    - request overrides
+   - request numeric overrides are presence-aware, so omitted values preserve the current effective value and explicit zero values override it
 6. verify required `api_key_env` environment variable:
    - missing/empty env value returns an invalid request with `ErrAPIKeyEnvMissing`
    - only the environment-variable name is retained; secret value is never returned
@@ -115,6 +116,14 @@ Validation content failures are not run errors:
 
 `Prepare` does not call the LLM.
 
+Runtime target notes:
+
+- Profile `extra_params` and request `extra_params` carry JSON-compatible values through prepared output, run metadata, and `domain.GenerateRequest.Target`.
+- The OpenAI-compatible client serializes non-empty `reasoning_effort` as a top-level provider request field.
+- The OpenAI-compatible client flattens `extra_params` into provider-specific top-level JSON request fields.
+- Empty `extra_params` keys, reserved outbound field names, and values that cannot be JSON-encoded fail before the provider request.
+- Resolved API-key values are never stored in `PreparedRun`, `RunResult`, logs, or HTTP responses.
+
 ## Run Flow
 
 `Run` performs: