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/frontmatter_test.go

@@ -0,0 +1,61 @@
+package api
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestParseFrontmatterSplitsHeaderAndBody(t *testing.T) {
+	src := "---\nwing: jepa-fx\nhall: facts\ncreated_at: 2026-01-01T00:00:00Z\n---\n# Title\n\nbody text\n"
+	fm, body := parseFrontmatter(src)
+
+	assert.Equal(t, "jepa-fx", fm.get("wing"))
+	assert.Equal(t, "facts", fm.get("hall"))
+	assert.Equal(t, "2026-01-01T00:00:00Z", fm.get("created_at"))
+	assert.Equal(t, "# Title\n\nbody text\n", body)
+}
+
+func TestParseFrontmatterNoHeader(t *testing.T) {
+	src := "# Just a body\n\nno frontmatter here\n"
+	fm, body := parseFrontmatter(src)
+
+	assert.Empty(t, fm.lines)
+	assert.Equal(t, src, body)
+}
+
+func TestFrontmatterSetOverridesExistingKey(t *testing.T) {
+	fm, _ := parseFrontmatter("---\nwing: a\nupdated_at: old\n---\nbody\n")
+	fm.set("updated_at", "new")
+
+	assert.Equal(t, "new", fm.get("updated_at"))
+	// No duplicate key.
+	assert.Equal(t, 1, strings.Count(fm.render(), "updated_at:"))
+}
+
+func TestFrontmatterSetAppendsNewKey(t *testing.T) {
+	fm, _ := parseFrontmatter("---\nwing: a\n---\nbody\n")
+	fm.set("supersedes", "abc123")
+
+	out := fm.render()
+	assert.Contains(t, out, "wing: a")
+	assert.Contains(t, out, "supersedes: abc123")
+}
+
+func TestFrontmatterRenderPreservesCustomFields(t *testing.T) {
+	src := "---\nwing: a\nhall: facts\ncustom_field: keep-me\ntags: [x, y]\n---\nbody\n"
+	fm, _ := parseFrontmatter(src)
+	fm.set("updated_at", "2026-06-22T00:00:00Z")
+
+	out := fm.render()
+	assert.Contains(t, out, "custom_field: keep-me")
+	assert.Contains(t, out, "tags: [x, y]")
+	assert.Contains(t, out, "updated_at: 2026-06-22T00:00:00Z")
+}
+
+func TestFrontmatterRenderRoundTrips(t *testing.T) {
+	src := "---\nwing: a\nhall: facts\n---\n"
+	fm, _ := parseFrontmatter(src)
+	assert.Equal(t, src, fm.render())
+}