Add AGENTS.md — onboarding for whoever picks this up next

Named by convention so a coding agent finds it without being told. Written to
get someone productive from a cold start without having to reconstruct the
reasoning from the diff.

Four sections carry the weight. The architecture rules that must not be broken,
each of which fails silently rather than loudly — intelligence never lives in
the API, authentication is not authorization, cost is charged against the full
commitment, sold and held are different things. The traps that have already
cost time here, with the specific symptom each produces: onConflictDoNothing
being a no-op without a constraint, z.coerce.boolean turning "false" into true,
grid children needing min-w-0, Drizzle emitting a cast Postgres rejects, the
CSP allowing exactly one inline script by hash, and the Prime Intellect API
quoting node totals rather than per-GPU prices. The conventions, including that
comments explain why rather than what. And an explicit start order.

The last section says what not to do, which is the part most easily lost: do
not copy component files from the MIT project we borrowed ideas from, do not
open self-registration on a shared identity provider, do not put a production
key on the CI runner, do not weaken the guard that refuses to serve the CRM
unauthenticated, and do not invent email addresses for real people.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -157,6 +157,10 @@ deploy/       Compose files and reverse-proxy snippets
 
 ## Documentation
 
+- **[AGENTS.md](./AGENTS.md) — start here if you are joining this codebase.**
+  Architecture rules, the traps that have already bitten, conventions, and
+  where to start.
+- [Build plan](./docs/build-plan.md) — what remains, in dependency order
 - [Ontology](./docs/ontology.md) — the domain model, and why it is shaped this way
 - [Seed data provenance](./docs/seed-data.md) — every claim, graded and cited
 - [Agent integration](./docs/agents.md) — Claude Code, Codex, prime-agent, Buzz