fix(ingest): merge existing frontmatter in writeHallNote instead of stacking (#86)

opts.Content can already carry its own "---\n...\n---" block (e.g. from
an upstream extraction/promotion step). writeHallNote used to prepend a
second block unconditionally, making a standard frontmatter parser blind
to everything the first block declared (title, tags, ...).

splitFrontmatter now pulls the existing block's fields out first;
wing/hall/created_at are always fresh-injected, and type/domain/
source_type prefer the note's own existing value over the opts
fallback. Remaining existing fields are carried through verbatim into
the single merged block.

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

ingestion/internal/api/handler.go

@@ -156,17 +156,40 @@ func writeHallNote(brainDir string, opts WriteNoteOptions) (string, error) {
 		return "", fmt.Errorf("create hall dir: %w", err)
 	}
 
+	existingFields, body := splitFrontmatter(opts.Content)
+	existingByKey := make(map[string]frontmatterField, len(existingFields))
+	for _, f := range existingFields {
+		existingByKey[f.key] = f
+	}
+	emitted := make(map[string]bool, 6)
+
 	var fm strings.Builder
 	fm.WriteString("---\n")
 	fmt.Fprintf(&fm, "wing: %s\n", brain.Sanitise(opts.Wing))
 	fmt.Fprintf(&fm, "hall: %s\n", opts.Hall)
 	fmt.Fprintf(&fm, "created_at: %s\n", time.Now().UTC().Format(time.RFC3339))
-	if opts.Type != "" {
+	emitted["wing"], emitted["hall"], emitted["created_at"] = true, true, true
-		fmt.Fprintf(&fm, "type: %s\n", opts.Type)
+
+	// writeField merges one key: opts.Content's own value (if the note already
+	// carries this field in its own frontmatter) always wins over the fallback,
+	// so promotion/extraction-step metadata survives verbatim instead of being
+	// shadowed by a second, stacked frontmatter block (#86).
+	writeField := func(key, fallback string) {
+		emitted[key] = true
+		if f, ok := existingByKey[key]; ok {
+			for _, line := range f.lines {
+				fm.WriteString(line)
+				fm.WriteString("\n")
 			}
-	if opts.Domain != "" {
+			return
-		fmt.Fprintf(&fm, "domain: %s\n", opts.Domain)
 		}
+		if fallback != "" {
+			fmt.Fprintf(&fm, "%s: %s\n", key, fallback)
+		}
+	}
+	writeField("type", opts.Type)
+	writeField("domain", opts.Domain)
+
 	sourceType := opts.SourceType
 	if sourceType == "" && opts.Hall == "facts" {
 		// Most hall=facts entries are first-party (an eval/benchmark the
@@ -175,18 +198,69 @@ func writeHallNote(brainDir string, opts WriteNoteOptions) (string, error) {
 		// citation-needing entry (brain-gardener#7).
 		sourceType = "internal"
 	}
-	if sourceType != "" {
+	writeField("source_type", sourceType)
-		fmt.Fprintf(&fm, "source_type: %s\n", sourceType)
+
+	for _, f := range existingFields {
+		if emitted[f.key] {
+			continue
+		}
+		for _, line := range f.lines {
+			fm.WriteString(line)
+			fm.WriteString("\n")
+		}
 	}
 	fm.WriteString("---\n")
 
-	if err := os.WriteFile(dest, []byte(fm.String()+opts.Content), 0o644); err != nil {
+	if err := os.WriteFile(dest, []byte(fm.String()+body), 0o644); err != nil {
 		return "", fmt.Errorf("write: %w", err)
 	}
 	rel, _ := filepath.Rel(brainDir, dest)
 	return filepath.ToSlash(rel), nil
 }
 
+// frontmatterField is one top-level YAML key from a frontmatter block,
+// along with its raw line and any indented continuation lines (e.g. a
+// bulleted list value spanning multiple lines).
+type frontmatterField struct {
+	key   string
+	lines []string
+}
+
+// splitFrontmatter splits a leading "---\n...\n---\n" YAML block out of
+// content, returning its top-level fields in original order and the
+// remaining body. If content has no leading frontmatter block, fields is
+// nil and body is content unchanged.
+func splitFrontmatter(content string) (fields []frontmatterField, body string) {
+	if !strings.HasPrefix(content, "---\n") {
+		return nil, content
+	}
+
+	lines := strings.Split(content, "\n")
+	i := 1
+	var cur *frontmatterField
+	for ; i < len(lines); i++ {
+		line := lines[i]
+		if strings.TrimSpace(line) == "---" {
+			i++
+			break
+		}
+		if line != "" && !strings.HasPrefix(line, " ") && !strings.HasPrefix(line, "\t") {
+			if cur != nil {
+				fields = append(fields, *cur)
+			}
+			key, _, _ := strings.Cut(line, ":")
+			cur = &frontmatterField{key: strings.TrimSpace(key), lines: []string{line}}
+		} else if cur != nil {
+			cur.lines = append(cur.lines, line)
+		}
+	}
+	if cur != nil {
+		fields = append(fields, *cur)
+	}
+	body = strings.Join(lines[i:], "\n")
+	return fields, body
+}
+
 // writeLegacyNote preserves the original brain/knowledge/ behaviour for
 // callers that have not adopted the wing/hall taxonomy.
 func writeLegacyNote(brainDir string, opts WriteNoteOptions) (string, error) {

ingestion/internal/api/handler_test.go

@@ -186,6 +186,48 @@ func TestWriteNote_HallRouteOmitsSourceTypeForNonFactsHalls(t *testing.T) {
 	assert.NotContains(t, string(got), "source_type")
 }
 
+func TestWriteNote_HallRouteMergesExistingFrontmatterInsteadOfStacking(t *testing.T) {
+	dir := t.TempDir()
+
+	rel, err := api.WriteNote(dir, api.WriteNoteOptions{
+		Content:  "---\ntitle: act_runner host-executor\ntags: [gitea-actions, act_runner]\n---\n\n# Body\n\nSome content.\n",
+		Filename: "act-runner-host-executor",
+		Wing:     "homelab",
+		Hall:     "failures",
+	})
+	require.NoError(t, err)
+
+	got, err := os.ReadFile(filepath.Join(dir, filepath.FromSlash(rel)))
+	require.NoError(t, err)
+	body := string(got)
+
+	// exactly one frontmatter block: only two "---" delimiter lines total
+	assert.Equal(t, 2, strings.Count(body, "---\n"), "expected a single merged frontmatter block, not stacked blocks")
+	assert.Contains(t, body, "wing: homelab")
+	assert.Contains(t, body, "hall: failures")
+	assert.Contains(t, body, "title: act_runner host-executor")
+	assert.Contains(t, body, "tags: [gitea-actions, act_runner]")
+	assert.Contains(t, body, "# Body")
+}
+
+func TestWriteNote_HallRouteExistingTypeWinsOverOptsType(t *testing.T) {
+	dir := t.TempDir()
+
+	rel, err := api.WriteNote(dir, api.WriteNoteOptions{
+		Content:  "---\ntype: hypothesis\n---\n\nBody.\n",
+		Filename: "note",
+		Wing:     "agentsquad",
+		Hall:     "decisions",
+		Type:     "decision", // should lose to content's own "type: hypothesis"
+	})
+	require.NoError(t, err)
+
+	got, err := os.ReadFile(filepath.Join(dir, filepath.FromSlash(rel)))
+	require.NoError(t, err)
+	assert.Contains(t, string(got), "type: hypothesis")
+	assert.NotContains(t, string(got), "type: decision")
+}
+
 func TestWrite_GeneratesFilenameIfAbsent(t *testing.T) {
 	dir, h := setup(t)
 	body, _ := json.Marshal(map[string]any{"content": "auto name"})