add script task templates and refactor convert-to-mp3 to use template pattern

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

seed/templates/standalone.sh

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# ── Standalone Template ──
+# For tasks that don't operate on a file/directory target.
+# All inputs are available as INPUT_<NAME> environment variables.
+#
+# Slot to fill before this template:
+#   main() { ... }    — the task logic, using $INPUT_* env vars
+
+# ── Execution ──
+main