feat(pipeline): per-wing sources, shared concepts/entities, one slug one page (#208)

With Config.Wing a source goes to wiki/<wing>/sources/ (wing/hall frontmatter);
concepts and entities stay in the shared wiki/concepts and wiki/entities, so an
entity cited from two wings is ONE page with both sources (the per-wing facts/
layout duplicated it). A source slug already used anywhere but its own wing's
sources/ is refused (slug is the graph identity). wiki.FindSlug added.
Mutation-checked: collision guard, shared inventory, per-wing source path.

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

ingestion/internal/pipeline/resolve.go

@@ -56,17 +56,12 @@ func normalizeTitle(s string) string {
 }
 
 // pageTypeOf is the page's type: from its directory in the retired layout, or
-// from `kind:` for a wing-layout facts page (concepts and entities share facts/).
+// "sources" for wiki/<wing>/sources/<slug>.md. Concepts and entities are always
+// in their shared directories.
 func pageTypeOf(p wiki.Page) wiki.PageType {
 	parts := strings.Split(filepath.ToSlash(p.Path), "/")
-	if len(parts) >= 4 && parts[0] == "wiki" {
-		switch parts[2] {
-		case "sources":
-			return wiki.PageTypeSource
-		case "facts":
-			pt, _ := wiki.KindFromContent(p.Content)
-			return pt
-		}
+	if len(parts) == 4 && parts[0] == "wiki" && parts[2] == "sources" {
+		return wiki.PageTypeSource
 	}
 	return pageTypeFromPath(p.Path)
 }