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/package.json

@@ -30,7 +30,7 @@
       "axios": "1.18.1"
     }
   },
-  "description": "Curriculum Project Hub — org-scoped Feishu collaboration and confined Agent runtime. Aligns to spec/System through ADR-0024.",
+  "description": "Curriculum Project Hub — org-scoped Feishu collaboration and confined Agent runtime. Semantics pinned by docs/adr/ (ADR-0001 through ADR-0027).",
   "scripts": {
     "dev": "npm run prisma:migrate && tsx watch src/server.ts",
     "build": "tsc -p tsconfig.json && npm run admin:build",