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/build.go

@@ -14,6 +14,13 @@ import (
 // (derived from the filename, not the LLM title). Pages whose title resolves to an
 // empty slug are skipped and returned as warnings instead.
 func BuildPages(rawPages []RawPage, sourceSlug, date string) ([]wiki.Page, []string) {
+	return BuildPagesIn("", rawPages, sourceSlug, date)
+}
+
+// BuildPagesIn is BuildPages for a wing: sources go to wiki/<wing>/sources/,
+// concepts and entities to wiki/<wing>/facts/ with `kind:` recording which.
+// An empty wing is the retired wiki/{sources,concepts,entities}/ layout.
+func BuildPagesIn(wing string, rawPages []RawPage, sourceSlug, date string) ([]wiki.Page, []string) {
 	out := make([]wiki.Page, 0, len(rawPages))
 	var warnings []string
 	for _, rp := range rawPages {
@@ -28,7 +35,7 @@ func BuildPages(rawPages []RawPage, sourceSlug, date string) ([]wiki.Page, []str
 			warnings = append(warnings, fmt.Sprintf("skipped page with empty title (type: %s)", rp.Type))
 			continue
 		}
-		out = append(out, buildPage(rp, sourceSlug, date))
+		out = append(out, buildPage(rp, sourceSlug, date, wing))
 	}
 	return out, warnings
 }
@@ -42,7 +49,7 @@ func computeSlug(rp RawPage, sourceSlug string) string {
 	return wiki.Slug(rp.Title)
 }
 
-func buildPage(rp RawPage, sourceSlug, date string) wiki.Page {
+func buildPage(rp RawPage, sourceSlug, date, wing string) wiki.Page {
 	var slug, dir string
 	switch rp.Type {
 	case "source":
@@ -59,8 +66,21 @@ func buildPage(rp RawPage, sourceSlug, date string) wiki.Page {
 		dir = "wiki/" + rp.Type
 	}
 
+	hall := ""
+	if wing != "" {
+		switch rp.Type {
+		case "source":
+			hall = "sources"
+		default:
+			hall = "facts"
+		}
+		dir = "wiki/" + wing + "/" + hall
+	}
 	path := dir + "/" + slug + ".md"
 	fm := buildFrontmatter(rp, date)
+	if wing != "" {
+		fm = addWingFrontmatter(fm, wing, hall, rp.Type)
+	}
 
 	return wiki.Page{
 		Path:    path,
@@ -122,6 +142,16 @@ func buildFrontmatter(rp RawPage, date string) string {
 	return sb.String()
 }
 
+// addWingFrontmatter inserts wing/hall (and kind for facts pages) right after
+// the opening fence of an already-built frontmatter block.
+func addWingFrontmatter(fm, wing, hall, rpType string) string {
+	extra := "wing: " + wing + "\nhall: " + hall + "\n"
+	if hall == "facts" {
+		extra += "kind: " + rpType + "\n"
+	}
+	return strings.Replace(fm, "---\n", "---\n"+extra, 1)
+}
+
 func yamlScalar(s string) string {
 	// A raw newline or CR would end the scalar for every line-oriented
 	// frontmatter reader and start a new key (PR #147 review: a title of