feat: markdown import splits H2 leafs in Go (#26)

Drop the Python ExecFile wrapper. Conversion still does not write
Ladybug; add a dry-run fixture check for the index adapter split.

internal/mdleaves/yaml.go

@@ -0,0 +1,36 @@
+package mdleaves
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+func EncodeYAML(leafs []Leaf) string {
+	if len(leafs) == 0 {
+		return "[]\n"
+	}
+	var b strings.Builder
+	for _, lf := range leafs {
+		b.WriteString("-\n")
+		writeKV(&b, "source", lf.Source)
+		writeKV(&b, "repo", lf.Repo)
+		writeKV(&b, "heading", lf.Heading)
+		writeKV(&b, "text", lf.Text)
+		writeKV(&b, "type", lf.Type)
+		writeKV(&b, "status", lf.Status)
+		writeKV(&b, "related", lf.Related)
+	}
+	return b.String()
+}
+
+func writeKV(b *strings.Builder, k, v string) {
+	fmt.Fprintf(b, "  %s: %s\n", k, yamlScalar(v))
+}
+
+func yamlScalar(s string) string {
+	if strings.Contains(s, "\n") || s == "" || strings.ContainsAny(s, ":#'\"[]{}&*!|>%@`") || s != strings.TrimSpace(s) {
+		return strconv.Quote(s)
+	}
+	return s
+}