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

.gitea/workflows/checker-check.yml

@@ -1,12 +1,12 @@
 name: checker check
 
 # Builds and lints the Rust implementation crates under crates/ (the rule-based
-# checker that "stands in Lean's position" at product runtime).
+# lesson checker).
 #
-# Like spec-check, this is an INTERNAL gate on the implementation's own health
-# (does it build, pass its tests, satisfy clippy + rustfmt?). It is NOT a
-# spec-to-implementation conformance gate — implementations align to the Lean
-# contract by human review, not by CI. See the repo README.
+# This is an INTERNAL gate on the implementation's own health
+# (does it build, pass its tests, satisfy clippy + rustfmt?). There is no
+# decision-to-implementation conformance gate — implementations align to the
+# ADRs by human review, not by CI. See the repo README.
 
 on:
   push: