fix(api): UpdateNote reserves updated_at/supersedes/supersede_reason from the caller's fragment (#137)

Closes #125. Skips updated_at/supersedes/supersede_reason in the caller fragment merge loop so they cannot be set via the fragment, only via the intended entry points.
delete_branch_after_merge=true

Co-authored-by: Mathias <mthbqv@gmail.com>

ingestion/internal/api/update_test.go

@@ -231,6 +231,45 @@ func TestUpdateThenReadRoundTripsHash(t *testing.T) {
 	assert.Equal(t, hash, readHash, "update content_hash must round-trip through ReadNote")
 }
 
+func TestUpdateNoteSupersedeReasonNotSettableViaCallerFragmentWhenReasonOptionEmpty(t *testing.T) {
+	// hyperguild#125, found in #111's review: the merge loop applied every
+	// field from the caller's leading frontmatter fragment, including
+	// supersede_reason -- which is only re-set unconditionally when
+	// opts.Reason is non-empty, so a caller could set it via its own
+	// fragment and have it survive even with Reason left empty, bypassing
+	// the one intended entry point for that field.
+	brainDir := t.TempDir()
+	rel := seedNote(t, brainDir, "wiki/a/facts/n.md", "---\nwing: a\nhall: facts\n---\nold\n")
+
+	_, _, _, err := UpdateNote(brainDir, UpdateNoteOptions{
+		Path:    rel,
+		Content: "---\nsupersede_reason: snuck in via the fragment\n---\nnew\n",
+		// Reason deliberately left empty.
+	})
+	require.NoError(t, err)
+
+	got, err := os.ReadFile(filepath.Join(brainDir, filepath.FromSlash(rel)))
+	require.NoError(t, err)
+	assert.NotContains(t, string(got), "supersede_reason",
+		"supersede_reason must not appear at all when opts.Reason is empty, regardless of what the fragment tried to set")
+}
+
+func TestUpdateNoteSupersedeReasonFromOptionStillWorks(t *testing.T) {
+	// Regression pin: the legitimate entry point (opts.Reason) must be
+	// completely unaffected by #125's fix.
+	brainDir := t.TempDir()
+	rel := seedNote(t, brainDir, "wiki/a/facts/n.md", "---\nwing: a\nhall: facts\n---\nold\n")
+
+	_, _, _, err := UpdateNote(brainDir, UpdateNoteOptions{
+		Path: rel, Content: "new\n", Reason: "facts changed",
+	})
+	require.NoError(t, err)
+
+	got, err := os.ReadFile(filepath.Join(brainDir, filepath.FromSlash(rel)))
+	require.NoError(t, err)
+	assert.Contains(t, string(got), "supersede_reason: facts changed")
+}
+
 func TestContentHashStable(t *testing.T) {
 	assert.Equal(t, ContentHash([]byte("abc")), ContentHash([]byte("abc")))
 	assert.NotEqual(t, ContentHash([]byte("abc")), ContentHash([]byte("abd")))