fix(ingest): thread source/author/published frontmatter through extraction (#85)

pipeline.Run parses source/author/published from the raw content's own
frontmatter and applies them deterministically to source-type RawPages
after LLM extraction — never relies on the LLM to copy them through.
buildFrontmatter now emits all three (source pages only) when present.

Backfill of the 46 already-affected wiki/sources/*.md notes is a
separate concern per the issue, not done here.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Roq1ajWKR5f1hG5Df9wC6A

ingestion/internal/pipeline/build.go

@@ -76,6 +76,15 @@ func buildFrontmatter(rp RawPage, date string) string {
 		}
 		fmt.Fprintf(&sb, "date_ingested: %s\n", date)
 		fmt.Fprintf(&sb, "last_updated: %s\n", date)
+		if rp.Source != "" {
+			fmt.Fprintf(&sb, "source: %s\n", yamlScalar(rp.Source))
+		}
+		if rp.Author != "" {
+			fmt.Fprintf(&sb, "author: %s\n", yamlScalar(rp.Author))
+		}
+		if rp.Published != "" {
+			fmt.Fprintf(&sb, "published: %s\n", yamlScalar(rp.Published))
+		}
 	case "concept":
 		if rp.Domain != "" {
 			fmt.Fprintf(&sb, "domain: %s\n", yamlScalar(rp.Domain))