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.go

@@ -110,9 +110,22 @@ func UpdateNote(brainDir string, opts UpdateNoteOptions) (relPath, contentHash,
 
 	fm, _ := parseFrontmatter(string(prior))
 	for _, l := range contentFM.lines {
-		if l.key != "" {
-			fm.set(l.key, l.value)
+		// updated_at/supersedes/supersede_reason are this function's own
+		// output, not something a caller's fragment sets — updated_at and
+		// supersedes are already unconditionally re-set below regardless of
+		// what the loop does with them, but supersede_reason is only
+		// re-set when opts.Reason is non-empty, so without this skip a
+		// caller could set it via its own fragment and have it survive
+		// even when Reason is empty, bypassing the one intended entry
+		// point for that field (hyperguild#125, found in #111's review).
+		// Skipping all three here rather than relying on the unconditional
+		// overwrites below to save the other two keeps the invariant
+		// explicit at the one place it can be silently broken by a future
+		// reordering, not implicit in call order.
+		if l.key == "" || l.key == "updated_at" || l.key == "supersedes" || l.key == "supersede_reason" {
+			continue
 		}
+		fm.set(l.key, l.value)
 	}
 	fm.set("updated_at", time.Now().UTC().Format(time.RFC3339))
 	fm.set("supersedes", priorHash)

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")))