fix(api): frontmatter parser must not misparse indented list items as keys

hyperguild#128, filed while fixing the same bug class in the sibling
internal/wiki package's parseFrontmatterBlock (hyperguild#126 review
round 3). This package was the original parseFrontmatter/set()/render()
shape that reimplementation was modeled on, and had the same gap: an
indented YAML block-list item containing a colon (e.g. a title with a
subtitle inside an aliases: list) got cut at the colon and treated as a
fake top-level key, since strings.Cut doesn't know a line is a
continuation just because TrimSpace(key) happens to be non-empty.
Re-rendering then stripped the item's indentation, de-indenting it out
of the list it belongs to. This package backs UpdateNote/writeHallNote,
so it's on the live brain_update write path.

Fixed the same way as the wiki package: check indentation before
calling Cut at all -- a line starting with a space or tab is a
continuation, full stop, regardless of what it contains.

Second, related bug found via the fix's own test fixture, not
speculatively: render() wrote "key: " (trailing space) for a key with
an empty value (the same "aliases:" block-list header line) instead of
bare "key:" -- a different byte sequence than the original, breaking
this type's whole documented purpose (a byte-preserving line view, not
a YAML re-marshal). No current caller sets an empty-value key via
set(), so this hadn't shown up in a set()-driven path, but it fires on
every ordinary parse-then-render round-trip of a note that already has
one -- confirmed directly via this fix's own test output before writing
a fix for it.

Flip-tested both independently: disabling either fix turns its
corresponding test red, the other stays green.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

ingestion/internal/api/frontmatter.go

@@ -40,6 +40,15 @@ func parseFrontmatter(src string) (frontmatter, string) {
 	body := rest[end+len("\n---\n"):]
 
 	for _, line := range strings.Split(block, "\n") {
+		// An indented line is a continuation (a YAML block-list item, a
+		// wrapped value) never a top-level key, regardless of whether it
+		// happens to contain a colon — "  - 'Chapter 3: The Reckoning'" is
+		// not a key named "- 'Chapter 3" (hyperguild#128). Checked before
+		// Cut so a colon inside one never reaches the key/value path.
+		if line != "" && (line[0] == ' ' || line[0] == '\t') {
+			fm.lines = append(fm.lines, fmLine{raw: line})
+			continue
+		}
 		key, val, ok := strings.Cut(line, ":")
 		key = strings.TrimSpace(key)
 		if !ok || key == "" || strings.HasPrefix(strings.TrimSpace(line), "#") {
@@ -83,9 +92,19 @@ func (f *frontmatter) render() string {
 	var b strings.Builder
 	b.WriteString("---\n")
 	for _, l := range f.lines {
-		if l.key == "" {
+		switch {
+		case l.key == "":
 			b.WriteString(l.raw)
-		} else {
+		case l.value == "":
+			// A key with nothing after the colon (the YAML block-list
+			// shape: "aliases:" followed by indented "- item" lines on
+			// their own raw lines) must stay bare — "key: " with a
+			// trailing space is a different byte sequence than the
+			// original, and this type's whole purpose (see the package
+			// doc) is a byte-preserving round-trip.
+			b.WriteString(l.key)
+			b.WriteByte(':')
+		default:
 			b.WriteString(l.key)
 			b.WriteString(": ")
 			b.WriteString(l.value)