router: add ActionResolutionMethod type and ResolvedBy field to Slots

Five disjoint values tracking which component selected the exact function:
grammar_fixed, grammar_matcher, extractor_raw, extractor_llm_text,
fallback_matcher. Slots.ResolvedBy carries provenance at the selection
point.

internal/dialogue/session.go

@@ -31,6 +31,10 @@ type Slots struct {
 	Fn      string
 	Args    []string
 	HasFn   bool
+	// ResolvedBy — which component selected the exact function. Mirrors
+	// router.Slots.ResolvedBy. Carried as a string because dialogue cannot
+	// import router (import cycle).
+	ResolvedBy string
 }
 
 // Turn represents one utterance in a multi-turn dialogue history.