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/pipeline.go

@@ -59,6 +59,8 @@ func Run(ctx context.Context, cfg Config, brainDir, content, source string, dryR
 		allWarnings = append(allWarnings, warnings...)
 	}
 
+	applySourceMeta(allRaw, parseContentFrontmatter(content))
+
 	return buildAndWrite(allRaw, sourceSlug, date, brainDir, source, inventory, allWarnings, dryRun)
 }