chore: remove Lean spec; ADRs are the single source of truth

The spec/ Lean semantic master had no conformance gate, no codegen, and
no CI tie to implementations — alignment was carried entirely by human
review, the same mechanism that carries the ADRs. In practice the ADRs
plus greppable code comments were already the load-bearing artifacts,
so spec/ was the most expensive kind of stale documentation.

- delete spec/ and the spec-check CI workflow
- README: constitution rewritten around ADRs as decision truth
- AGENTS.md/CLAUDE.md: discipline re-anchored (new decisions -> new ADR,
  never rewrite ADR history; supersede instead)
- code comments: re-anchor 'Mirrors Spec.X' invariants to ADR numbers
  (cph-diag, cph-check, cph-model, hub runner/capacity/org, prisma)
- leave ADR bodies and .scratch audit snapshots untouched (history);
  fix live references in open readiness tickets

hub/src/org/members.ts

@@ -1,7 +1,7 @@
 /**
  * Organization membership management for org admin (ADR-0021).
  *
- * Role rules (product pin, not yet in Lean):
+ * Role rules (product pin, not yet recorded in an ADR):
  * 1. Actor must be OWNER or ADMIN (enforced at HTTP layer).
  * 2. Only OWNER can grant/revoke OWNER or modify another OWNER.
  * 3. Cannot revoke or demote the last remaining OWNER.