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/prisma/migrations/20260714120000_drop_legacy_platform_role_assignment/migration.sql

@@ -1,6 +1,6 @@
 -- ADR-0023 rejected the legacy `PlatformRoleAssignment` / `PlatformRole`{ADMIN,TEACHER}
 -- model: the platform administration control plane is a separate identity/session/
--- audit surface (see `Spec.System.PlatformAdministration`), intentionally not built
+-- audit surface, intentionally not built
 -- in alpha (ADR-0025, `hub/deploy/README.md`). The legacy table has no runtime
 -- reader — no guard, route, or service queries it for an authorization decision —
 -- and ADR-0023 requires it to be migrated/replaced before the platform panel ships.