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

crates/README.md

@@ -1,7 +1,8 @@
 # crates/
 
-These crates implement the rule-based lesson checker that aligns to the
-semantic master in `spec/`: it reads an engineering-file (one lesson, ADR-0005)
+These crates implement the rule-based lesson checker whose semantics are
+pinned by the ADRs in `docs/adr/`: it reads an engineering-file (one lesson,
+ADR-0005)
 laid out per ADR-0008 (declarative `manifest.toml` + per-element
 `element.toml`), validates structure and content, and emits diagnostics.
 `cph-diag` (the shared diagnostic vocabulary), `cph-model` (the ADR-0008 loader),