feat(llm): ReasoningEffort request option and ErrUnsupported sentinel

Groundwork for the provider phase: reasoning levels map to native knobs
(OpenAI reasoning_effort, Ollama think); ErrUnsupported marks declared
capability mismatches that chains advance past without health penalty.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

llm/errors.go

@@ -26,6 +26,13 @@ const (
 // condition. Chains advance past it without penalizing the target's health.
 var ErrModelNotFound = errors.New("model not found")
 
+// ErrUnsupported marks a request the target cannot serve by declaration —
+// e.g. images that cannot be normalized to its capabilities, or a feature
+// (tools, structured output) it does not support. Permanent for the target,
+// but chains advance past it without penalizing health: another element may
+// well be able to serve the request.
+var ErrUnsupported = errors.New("request unsupported by target")
+
 // APIError is a structured provider error carrying enough context to
 // classify it and to debug it.
 type APIError struct {
@@ -96,7 +103,7 @@ func Classify(err error) ErrorClass {
 	if errors.Is(err, context.DeadlineExceeded) {
 		return ClassTransient
 	}
-	if errors.Is(err, ErrModelNotFound) {
+	if errors.Is(err, ErrModelNotFound) || errors.Is(err, ErrUnsupported) {
 		return ClassPermanent
 	}
 	if errors.Is(err, syscall.ECONNREFUSED) || errors.Is(err, syscall.ECONNRESET) {