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/hub-check.yml

@@ -1,8 +1,8 @@
 name: hub check
 
 # Builds, type-checks, and tests the Hub TS package under hub/.
-# The Hub is the Feishu-group collaboration + agent runtime half
-# (spec/System implementation). This is an INTERNAL gate on the Hub's own
+# The Hub is the Feishu-group collaboration + agent runtime half.
+# This is an INTERNAL gate on the Hub's own
 # health, like checker-check is for the Rust half.
 
 on: