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_test.go

@@ -130,6 +130,40 @@ func TestRun_MergesDuplicatePaths(t *testing.T) {
 	assert.Contains(t, string(content), "[[Baz]]")
 }
 
+func TestRun_ThreadsSourceAuthorPublishedFromContentFrontmatter(t *testing.T) {
+	brainDir := t.TempDir()
+	for _, sub := range []string{"wiki/concepts", "wiki/entities", "wiki/sources"} {
+		require.NoError(t, os.MkdirAll(filepath.Join(brainDir, sub), 0o755))
+	}
+
+	llmResponse := mustJSON([]RawPage{{
+		Title:   "Ornith",
+		Type:    "source",
+		Subtype: "article",
+		Content: "## Summary\n\nAn agentic coding model.\n",
+	}})
+
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		_ = json.NewEncoder(w).Encode(map[string]any{
+			"choices": []map[string]any{{"message": map[string]any{"content": llmResponse}}},
+		})
+	}))
+	defer srv.Close()
+
+	cfg := Config{Complete: llm.New(srv.URL, "", "m", 30*time.Second).Complete}
+	rawContent := "---\nsource: https://example.com/ornith\nauthor: Jane Doe\npublished: 2026-07-20\n---\n\nAn agentic coding model that runs on your laptop.\n"
+
+	result, err := Run(context.Background(), cfg, brainDir, rawContent, "ornith", false)
+	require.NoError(t, err)
+	require.Len(t, result.Pages, 1)
+
+	content, err := os.ReadFile(filepath.Join(brainDir, "wiki", "sources", "ornith.md"))
+	require.NoError(t, err)
+	assert.Contains(t, string(content), "source: 'https://example.com/ornith'")
+	assert.Contains(t, string(content), "author: 'Jane Doe'")
+	assert.Contains(t, string(content), "published: '2026-07-20'")
+}
+
 func mustJSON(v any) string {
 	b, err := json.Marshal(v)
 	if err != nil {