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_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"})