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/pi.ts

@@ -47,20 +47,28 @@ export interface PromptableSession extends AgentSession {
 }
 
 function getSessionStateError(session: AgentSession): string {
-  const error = (session as any).state?.error;
+  const state = (session as any).state;
+  const error = state?.errorMessage ?? state?.error;
   return typeof error === "string" ? error : "";
 }
 
 function clearSessionStateError(session: AgentSession): void {
   const state = (session as any).state;
-  if (!state || typeof state !== "object" || !("error" in state)) {
+  if (!state || typeof state !== "object") {
     return;
   }
 
-  try {
-    state.error = undefined;
-  } catch {
-    // Best effort only. Some session implementations may expose readonly state.
+  // pi-coding-agent 0.70+ exposes `errorMessage` as readonly — writes are
+  // silently ignored. Pre-0.70 used mutable `state.error`. Best-effort clear
+  // both so transcripts carry forward to the next prompt cleanly.
+  for (const key of ["errorMessage", "error"]) {
+    if (key in state) {
+      try {
+        state[key] = undefined;
+      } catch {
+        // readonly — no-op
+      }
+    }
   }
 }