packs: labelle add pack/feature scaffold (recipe shares the manifest template source) #271

labelle add pack/feature scaffold (Phase 5 · optional sugar).

Follows the existing labelle init template pattern (init_cmd.zig):

• labelle add pack <name> — scaffolds packs/<name>/{components,events,scripts,prefabs,hooks}/ + pack.labelle.
• labelle add feature <kind> <name> — need/role/status, hardcoded behind an explicit --kind flag (enum/registry inference is the trap).

The scaffold's step template is also the manifest's "recipe" source (assembler #manifest) — one definition. Not load-bearing; the convention + manifest carry the value without it.

Part of Packs — RFC Flying-Platform/flying-platform-labelle#561 · umbrella labelle-toolkit/labelle-engine#651 · epic labelle-toolkit/labelle-engine#650.