feat(pipeline): write into wiki/<wing>/{sources,facts}/ when Config.Wing is set (#205 S2)

Sources -> wiki/<wing>/sources, concepts/entities -> wiki/<wing>/facts with
kind: frontmatter; inventory, title resolution and back-references are
confined to the target wing. Empty Wing keeps the retired layout unchanged.
Mutation-checked: widening the inventory beyond the wing fails the
cross-wing back-reference test; a non-plain wing name is rejected.

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

ingestion/internal/pipeline/resolve.go

@@ -32,7 +32,7 @@ func Resolve(proposed []wiki.Page, inventory map[wiki.PageType][]wiki.Entry) []w
 
 	out := make([]wiki.Page, 0, len(proposed))
 	for _, page := range proposed {
-		pt := pageTypeFromPath(page.Path)
+		pt := pageTypeOf(page)
 		title := extractTitle(page.Content)
 		k := key{pt: pt, normalized: normalizeTitle(title)}
 		if canonicalSlug, ok := lookup[k]; ok {
@@ -55,6 +55,22 @@ func normalizeTitle(s string) string {
 	return strings.Join(strings.Fields(s), " ")
 }
 
+// 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/).
+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
+		}
+	}
+	return pageTypeFromPath(p.Path)
+}
+
 // pageTypeFromPath extracts the wiki.PageType from a path like "wiki/entities/foo.md".
 func pageTypeFromPath(path string) wiki.PageType {
 	parts := strings.Split(filepath.ToSlash(path), "/")