fix(ingestion): decide quoting with YAML itself, not a colon heuristic (#90)

`title: "Foo" — bar` opens with a quote and is not quoted; the previous
prefix check read it as already-quoted and left two notes broken. Probing
the value as the right-hand side of a mapping answers the real question —
does this load as written — and leaves anything that already parses alone,
whatever it parses into.

Repairs the last two quotable files in mathias/brain. Only wiki/index.md
remains unparseable, for an unrelated reason: a truncated UTF-8 sequence.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BeAp5LTscnz2W7Ubt4eJ6m

ingestion/internal/frontmatter/frontmatter_test.go

@@ -90,6 +90,17 @@ func TestFix(t *testing.T) {
 			wantKey:     "title",
 			wantValue:   "Don't do this: it breaks",
 		},
+		{
+			// The value opens with a double quote and then keeps going after
+			// the closing one, so YAML reads it as a quoted scalar followed by
+			// junk. Treating "starts with a quote" as "already quoted" leaves
+			// this broken forever.
+			name:        "quotes a value that only looks quoted",
+			doc:         "---\ntitle: \"Evaluation Driven Generation\" — a TDD-style scaffold\ntags: [a, b]\n---\n\nbody\n",
+			wantChanged: true,
+			wantKey:     "title",
+			wantValue:   "\"Evaluation Driven Generation\" — a TDD-style scaffold",
+		},
 		{
 			name:        "leaves an already-valid document untouched",
 			doc:         "---\ntitle: 'Already quoted: fine'\nwing: homelab\n---\n\nbody\n",
@@ -129,6 +140,25 @@ func TestFix(t *testing.T) {
 	}
 }
 
+func TestFix_LeavesStructuredValuesStructured(t *testing.T) {
+	// Only the broken scalar may be touched. A flow sequence on a neighbouring
+	// line still has to load as a sequence afterwards, not as a string.
+	doc := "---\ntitle: Broken: here\ntags: [prompting, llm]\ncount: 3\n---\n\nbody\n"
+
+	got, changed, err := Fix([]byte(doc))
+	require.NoError(t, err)
+	require.True(t, changed)
+
+	b, ok := block(got)
+	require.True(t, ok)
+	var m map[string]any
+	require.NoError(t, yaml.Unmarshal(b, &m))
+
+	assert.Equal(t, "Broken: here", m["title"])
+	assert.Equal(t, []any{"prompting", "llm"}, m["tags"])
+	assert.Equal(t, 3, m["count"])
+}
+
 func TestFix_PreservesBodyExactly(t *testing.T) {
 	body := "\nThe body has: colons, 'quotes', and\n---\na horizontal rule.\n"
 	doc := "---\ntitle: Broken: title\n---" + body