fix: adapt to AgentState.error → errorMessage rename

pi-coding-agent 0.70 replaced the mutable \`AgentState.error\` field with
a readonly \`AgentState.errorMessage\`. \`session.prompt()\` still does
not throw when retries are exhausted, so we still need to re-raise the
stored error after each prompt.

- checkSessionError (usage-limit-detector): widen parameter to accept
  either key; prefer errorMessage so new sessions work, fall back to
  error so we can deploy without forcing everyone's caches to rebuild.
- agent-reflection: same widening at the call site.
- pi.ts helpers: read both keys, best-effort clear both (the new field
  is readonly, so the write is a no-op on 0.70 sessions but still
  matters for mock sessions in tests).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/engine/src/agent-reflection.ts

@@ -111,8 +111,10 @@ export class AgentReflectionService {
       try {
         await promptWithFallback(session, this.buildReflectionPrompt(context, options.triggerDetail));
 
-        if (session.state?.error) {
-          throw new Error(session.state.error);
+        const sessionError = (session.state as { errorMessage?: string; error?: string } | undefined);
+        const stateErr = sessionError?.errorMessage ?? sessionError?.error;
+        if (stateErr) {
+          throw new Error(stateErr);
         }
       } finally {
         try {