feat(invariants): dev-mode event-contract assertions + session-log freeze (RFC 005 pt 3, RFC 008)

New @deepseek-ai/dsh-invariants plugin (pure listeners, off in prod) asserts
the event taxonomy at runtime — seq monotonicity, turn/step nesting, a
tool/result needs a prior tool/call (NOT the converse), legal agent/status
transitions — and deep-freezes logged event data so mutating history throws.
Seeded sessions are checked + frozen on session/created.

The real RFC 008 fix is always-on: deriveMessages now structured-clones the
content it emits, so the loop's sanctioned request/adapter mutation can no
longer reach back and rewrite the append-only log. The pervasive
DeepReadonly<T> type flip is rejected (compile-only, high-noise, castable) —
recorded in ADR 0012, which folds in RFC 008. Wired into both demos.

examples/echo-agent/cordis.yml

@@ -27,6 +27,11 @@
 - id: agents
   name: '@deepseek-ai/dsh-agent'
 
+# Dev-mode event-contract assertions + session-log freeze (off in prod;
+# on here so the demo smoke test exercises the contract).
+- id: invariants
+  name: '@deepseek-ai/dsh-invariants'
+
 - id: agent-loop
   name: '@deepseek-ai/dsh-agent-loop'
   config: