Align script rules with annotated script files

Scripts are now a first-class concept across fabrik manifests and commands. The script rule is the canonical script model, with [target.script] and shared field names so inline scripts and annotated script files use the same configuration surface. fabrik run and fabrik exec are script-aware, executing annotated script files via their shebang and auto-detecting the runtime, so fabrik exec -- <runtime> <script> no longer makes you remember (or repeat) the interpreter that the script already declares.