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/cph-check/src/lib.rs

@@ -19,13 +19,11 @@ const DEFAULT_TARGET: &str = "student";
 
 /// Severity of the render-coverage ("element ignored under a target") diagnostic.
 ///
-/// **PINNED to `warning` by the contract.** Mirrors the Lean master's
-/// `Spec.Courseware.renderIgnoredSeverity : Severity := .warning`
-/// (`spec/Spec/Courseware/Check/Diagnostic.lean`), itself citing ADR-0005: when a
+/// **PINNED to `warning` by ADR-0005:** when a
 /// `(kind, target)` pair has no render rule the checker reports that the element
 /// is ignored under that target and **does not block the export**. Naming the
-/// severity as a const makes "it is a warning, not an error" a greppable,
-/// alignable fact rather than an inline literal.
+/// severity as a const makes "it is a warning, not an error" a greppable
+/// fact rather than an inline literal.
 const RENDER_IGNORED_SEVERITY: Severity = Severity::Warning;
 
 /// The result of running [`check`] (or the check phases of [`build`]).
@@ -57,12 +55,12 @@ impl CheckReport {
 
     /// Whether any collected diagnostic is `Error`-severity.
     ///
-    /// **Legality decision (spec alignment).** `!has_errors()` is the
-    /// implementation of `Spec.Courseware.Legal` (`spec/Spec/Courseware/Check/Diagnostic.lean`):
-    /// a lesson is *legal* iff its diagnostics contain no error-level diagnostic
-    /// (warnings are non-blocking — see `Severity` / ADR-0010). There is no CI
-    /// gate enforcing this alignment (repo constitution); it is kept greppable
-    /// here so a reviewer can tie the orchestrator's gate to the Lean master.
+    /// **Legality decision (ADR-0010).** `!has_errors()` decides lesson
+    /// legality: a lesson is *legal* iff its diagnostics contain no error-level
+    /// diagnostic (warnings are non-blocking — see `Severity`). There is no CI
+    /// gate enforcing ADR↔implementation alignment (repo constitution); it is
+    /// kept greppable here so a reviewer can tie the orchestrator's gate to
+    /// the ADR.
     pub fn has_errors(&self) -> bool {
         self.diagnostics
             .iter()