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/request.go

@@ -37,6 +37,12 @@ type Request struct {
 
 	// StopSequences halt generation when emitted.
 	StopSequences []string
+
+	// ReasoningEffort requests a reasoning/thinking level from models that
+	// support one: "low", "medium", or "high" (empty = provider default).
+	// Providers map it to their native knob (OpenAI reasoning_effort,
+	// Ollama think levels) and ignore it where no mapping exists.
+	ReasoningEffort string
 }
 
 // Option mutates a Request before it is sent. Options passed to Generate or
@@ -88,6 +94,12 @@ func WithStopSequences(stops ...string) Option {
 	return func(r *Request) { r.StopSequences = stops }
 }
 
+// WithReasoningEffort requests a reasoning/thinking level ("low", "medium",
+// "high") from models that support one.
+func WithReasoningEffort(level string) Option {
+	return func(r *Request) { r.ReasoningEffort = level }
+}
+
 // Apply returns a copy of the request with all options applied. Providers
 // and wrappers call this once at the top of Generate/Stream.
 func (r Request) Apply(opts ...Option) Request {