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/08-prove-production-release-gate.md

@@ -7,6 +7,6 @@ Blocked by: 01, 02, 03, 04, 05, 06, 07, 09, 10, 11, 12, 13, 14, 15, 16, 17, 18,
 ## Question
 
 After the readiness investigations and resulting fixes are resolved, can one
-repeatable release procedure prove build/test/spec health, deploy a clean
+repeatable release procedure prove build/test health, deploy a clean
 production-like environment, exercise critical tenant and agent journeys,
 verify observability and recovery, and either roll forward or roll back safely?