fix(KB-153): detect exhausted-retry errors from pi-coding-agent sessions

- Add checkSessionError helper that re-raises errors stored on session.state.error after prompt() resolves silently when retries are exhausted
- Integrate checkSessionError in executor, triage, merger, and reviewer agents so existing catch blocks with isUsageLimitError can trigger UsageLimitPauser
- Add tests for checkSessionError and for each agent's error propagation path
- Add audit report documenting the error propagation gap
- Add changeset for the fix

packages/engine/src/triage.ts

@@ -6,7 +6,7 @@ import { createKbAgent } from "./pi.js";
 import { PRIORITY_SPECIFY, type AgentSemaphore } from "./concurrency.js";
 import { AgentLogger } from "./agent-logger.js";
 import { triageLog } from "./logger.js";
-import { isUsageLimitError, type UsageLimitPauser } from "./usage-limit-detector.js";
+import { isUsageLimitError, checkSessionError, type UsageLimitPauser } from "./usage-limit-detector.js";
 
 const TRIAGE_SYSTEM_PROMPT = `You are a task specification agent for "kb", an AI-orchestrated task board.
 
@@ -366,6 +366,9 @@ export class TriageProcessor {
           const agentPrompt = buildSpecificationPrompt(detail, promptPath, settings, attachmentContents);
           await session.prompt(agentPrompt, imageContents.length > 0 ? { images: imageContents } : undefined);
 
+          // Re-raise errors that pi-coding-agent swallowed after exhausting retries.
+          checkSessionError(session);
+
           // Check if the agent flagged a duplicate
           const { readFile } = await import("node:fs/promises");
           const { join } = await import("node:path");