feat(cli): aura new scaffold module — validation, templates, emit (0103 task 1)
Token-replace templates (Cargo.toml/Aura.toml/.gitignore/lib.rs/
signal.json/CLAUDE.md) parameterized by name/namespace/engine root;
argv validation (usage exit-2 class), refusals before first write
(existing destination, missing engine root with --engine-path hint),
best-effort git init. Unit tests: validation table, snake derivation,
engine-root default, token-free rendering, refusal pair — 5/5 green.
Verified against the fixed plan (derive(Debug) — see the plan-fix
commit); CLI wiring follows as task 2.
refs #180
Brummel
01500cba5e