Lab Zero v1.19.1 — template purpose scaffolding cleanup

Template cleanup — nothing changes for existing projects.

The project template's "What this is" section carried a leftover instruction telling you to
write a purpose paragraph that new-project.sh had already filled in from your /kickoff
answer. Removed.

• New projects only. AGENTS.md is part of your personal layer, so update.sh never
  overwrites it — existing workspaces are unaffected and need no action.
• No machinery, script, or hook changed. Constitution-drift baselines are byte-identical, so
  no existing project gains a drift warning.