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

.scratch/saas-production-readiness/issues/05-define-abuse-capacity-controls.md

@@ -29,7 +29,7 @@ workload brakes.
 
 The full current-state inventory, accepted behavior, and release evidence are
 recorded in [Initial abuse and capacity controls](../assets/initial-abuse-capacity-controls.md),
-with the durable decision in ADR-0022 and `Spec.System.Capacity`. Numerical
+with the durable decision in ADR-0022. Numerical
 ceilings remain open until production-like calibration.
 
 The implementation frontier is: