feat(brain): add UpdateNote/ReadNote supersede primitives + frontmatter editor

Implements the api-layer half of #45. UpdateNote supersedes a note in
place (whole-note body replace, frontmatter re-stamp: updated_at,
supersedes=prior content hash, supersede_reason), preserving created_at,
wing, hall, and any custom fields. Never creates — a missing target is
an error so callers fall back to brain_write. ReadNote is the read-after-
write primitive (frontmatter + body + content_hash). ContentHash is the
sha256 handle that round-trips write/update → get.

Frontmatter is edited via a line-preserving ordered editor rather than a
yaml.v3 round-trip, which would reorder keys and strip comments — the
brain writes flat key:value frontmatter by hand.

Embeddings are not refreshed here: the rewritten file's mtime advances,
which the out-of-band vectorstore.Sync ticker uses to re-embed it — the
same mechanism brain_write relies on.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ingestion/internal/api/update_test.go

@@ -0,0 +1,130 @@
+package api
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// seedNote writes a note directly to disk and returns its relPath.
+func seedNote(t *testing.T, brainDir, rel, content string) string {
+	t.Helper()
+	full := filepath.Join(brainDir, filepath.FromSlash(rel))
+	require.NoError(t, os.MkdirAll(filepath.Dir(full), 0o755))
+	require.NoError(t, os.WriteFile(full, []byte(content), 0o644))
+	return rel
+}
+
+func TestUpdateNoteSupersedesAndStamps(t *testing.T) {
+	brainDir := t.TempDir()
+	rel := seedNote(t, brainDir, "wiki/jepa-fx/facts/val-vol.md",
+		"---\nwing: jepa-fx\nhall: facts\ncreated_at: 2026-01-01T00:00:00Z\ncustom: keep-me\n---\n# Old\n\nold body\n")
+
+	relPath, hash, priorHash, err := UpdateNote(brainDir, UpdateNoteOptions{
+		Path:    rel,
+		Content: "# New\n\nnew body\n",
+		Reason:  "facts changed",
+	})
+	require.NoError(t, err)
+	assert.Equal(t, rel, relPath)
+	assert.NotEmpty(t, hash)
+	assert.NotEmpty(t, priorHash)
+	assert.NotEqual(t, hash, priorHash)
+
+	got, err := os.ReadFile(filepath.Join(brainDir, filepath.FromSlash(rel)))
+	require.NoError(t, err)
+	s := string(got)
+	// Body replaced.
+	assert.Contains(t, s, "# New")
+	assert.NotContains(t, s, "old body")
+	// Prior fields preserved.
+	assert.Contains(t, s, "wing: jepa-fx")
+	assert.Contains(t, s, "hall: facts")
+	assert.Contains(t, s, "created_at: 2026-01-01T00:00:00Z")
+	assert.Contains(t, s, "custom: keep-me")
+	// Supersession stamped.
+	assert.Contains(t, s, "updated_at:")
+	assert.Contains(t, s, "supersedes: "+priorHash)
+	assert.Contains(t, s, "supersede_reason: facts changed")
+}
+
+func TestUpdateNoteResolvesByWingHallSlug(t *testing.T) {
+	brainDir := t.TempDir()
+	seedNote(t, brainDir, "wiki/jepa-fx/facts/val-vol.md",
+		"---\nwing: jepa-fx\nhall: facts\n---\nold\n")
+
+	relPath, _, _, err := UpdateNote(brainDir, UpdateNoteOptions{
+		Wing: "jepa-fx", Hall: "facts", Slug: "val-vol",
+		Content: "new\n",
+	})
+	require.NoError(t, err)
+	assert.Equal(t, "wiki/jepa-fx/facts/val-vol.md", relPath)
+}
+
+func TestUpdateNoteErrorsOnMissingAndDoesNotCreate(t *testing.T) {
+	brainDir := t.TempDir()
+
+	_, _, _, err := UpdateNote(brainDir, UpdateNoteOptions{
+		Wing: "jepa-fx", Hall: "facts", Slug: "ghost",
+		Content: "x\n",
+	})
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "does not exist")
+
+	// No file created.
+	_, statErr := os.Stat(filepath.Join(brainDir, "wiki/jepa-fx/facts/ghost.md"))
+	assert.True(t, os.IsNotExist(statErr), "missing-target update must not create a note")
+}
+
+func TestUpdateNoteRejectsTraversal(t *testing.T) {
+	brainDir := t.TempDir()
+	_, _, _, err := UpdateNote(brainDir, UpdateNoteOptions{
+		Path:    "../escape.md",
+		Content: "x\n",
+	})
+	require.Error(t, err)
+}
+
+func TestReadNoteReturnsFrontmatterBodyHash(t *testing.T) {
+	brainDir := t.TempDir()
+	rel := seedNote(t, brainDir, "wiki/jepa-fx/facts/n.md",
+		"---\nwing: jepa-fx\nhall: facts\n---\n# Body\n\ntext\n")
+
+	fm, body, hash, err := ReadNote(brainDir, rel)
+	require.NoError(t, err)
+	assert.Equal(t, "jepa-fx", fm["wing"])
+	assert.Equal(t, "facts", fm["hall"])
+	assert.Equal(t, "# Body\n\ntext\n", body)
+
+	// Hash matches ContentHash of the raw bytes on disk (round-trip).
+	raw, _ := os.ReadFile(filepath.Join(brainDir, filepath.FromSlash(rel)))
+	assert.Equal(t, ContentHash(raw), hash)
+}
+
+func TestReadNoteRejectsTraversal(t *testing.T) {
+	brainDir := t.TempDir()
+	_, _, _, err := ReadNote(brainDir, "../../etc/passwd")
+	require.Error(t, err)
+}
+
+func TestUpdateThenReadRoundTripsHash(t *testing.T) {
+	brainDir := t.TempDir()
+	rel := seedNote(t, brainDir, "wiki/a/facts/n.md", "---\nwing: a\nhall: facts\n---\nold\n")
+
+	_, hash, _, err := UpdateNote(brainDir, UpdateNoteOptions{Path: rel, Content: "new\n"})
+	require.NoError(t, err)
+
+	_, _, readHash, err := ReadNote(brainDir, rel)
+	require.NoError(t, err)
+	assert.Equal(t, hash, readHash, "update content_hash must round-trip through ReadNote")
+}
+
+func TestContentHashStable(t *testing.T) {
+	assert.Equal(t, ContentHash([]byte("abc")), ContentHash([]byte("abc")))
+	assert.NotEqual(t, ContentHash([]byte("abc")), ContentHash([]byte("abd")))
+	assert.True(t, strings.HasPrefix(ContentHash([]byte("")), "")) // hex, non-panicking
+}