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>

majordomo.go

@@ -76,6 +76,9 @@ const (
 // ErrModelNotFound re-exports llm.ErrModelNotFound.
 var ErrModelNotFound = llm.ErrModelNotFound
 
+// ErrUnsupported re-exports llm.ErrUnsupported.
+var ErrUnsupported = llm.ErrUnsupported
+
 // Re-exported content and message constructors.
 func Text(s string) Part                              { return llm.Text(s) }
 func Image(mime string, data []byte) Part             { return llm.Image(mime, data) }
@@ -96,6 +99,7 @@ func WithTemperature(t float64) Option                  { return llm.WithTempera
 func WithTopP(p float64) Option                         { return llm.WithTopP(p) }
 func WithMaxTokens(n int) Option                        { return llm.WithMaxTokens(n) }
 func WithStopSequences(stops ...string) Option          { return llm.WithStopSequences(stops...) }
+func WithReasoningEffort(level string) Option           { return llm.WithReasoningEffort(level) }
 
 // WithModelCapabilities re-exports llm.WithCapabilities for Provider.Model
 // calls made through this package.