feat(wing): Router — rules-only placement: deliberate subfolder, else default wing (#208)

No model. A subfolder/frontmatter wing must be a plain name of an existing wing
and then places; conflicting signals and client-marker text park; everything
else goes to the default wing. Wings are read from disk per decision.
Mutation-checked.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

ingestion/internal/wing/resolve.go

@@ -20,6 +20,7 @@ type Source string
 
 const (
 	Explicit   Source = "explicit"   // subfolder or frontmatter named the wing
+	Default    Source = "default"    // no signal; the configured default wing
 	Classified Source = "classified" // classifier + scorer agreed, confident
 	Exception  Source = "exception"  // needs a human
 )