Refuse to run on config keys the builder cannot build

build_demo.py reads every key with cfg.get(), so a config describing objects it does
not implement was skipped in silence — it printed DONE over an empty database. That
is the same "reports success over an environment that isn't" failure the purge section
exists to prevent, one layer up, and it is how a vertical config (take-offs, progress
statements) would have failed.

Unknown top-level keys now list themselves and abort. DEMO_ALLOW_UNKNOWN_KEYS=1
overrides for a deliberate partial run. Keys starting with _ stay free for comments,
so the shipped example config passes clean.

SKILL.md now states outright that build_demo.py is a per-engagement template rather
than an engine, since that is what makes the failure mode surprising.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

SKILL.md

@@ -146,7 +146,16 @@ are heavy; allow several minutes.
 Industry-appropriate, minimal but believable, generated from the brief — the client's own
 partners, products, projects and orders, staged so each hero has a record to fire on.
 Drive it from a `demo_config.json` (see `demo_config.example.json`) and
-`scripts/build_demo.py`. Rules:
+`scripts/build_demo.py`.
+
+**`build_demo.py` is a per-engagement template, not an engine.** It implements one shape
+(products, fleet, credit limits, prestaged receivables) and reads every key with
+`cfg.get()`, so anything it does not implement would otherwise be skipped in silence — a
+`DONE` over an empty database. It now refuses to run on config keys it cannot build, listing
+them; extend the script, or drop the keys. `DEMO_ALLOW_UNKNOWN_KEYS=1` overrides for a
+deliberate partial run. Extend `KNOWN_KEYS` whenever you add a block.
+
+Rules:
 
 - **Get-or-create by name** so re-runs are idempotent.
 - Set the right **currency** first (activate it with `context={'active_test': False}`, set