fix(llm): classify empty model completions as retryable EMPTY_RESPONSE

A well-formed provider stream that ends with finish_reason stop and zero
content blocks previously became a successful empty assistant message: the
turn completed silently, and drivers like goal-session counted the no-op
round. Both adapters now map that degenerate completion to a finish
{kind:'error'} with the new canonical EMPTY_RESPONSE code from dsh-llm, and
dsh-llm-retry adds the code to its default retryable set, so the existing
closed-step recovery path retries it and fails loud once the budget is
exhausted.

Covered by adapter unit tests, an llm-retry default-policy test, and a new
authored keyless ACP snapshot (empty-response-retry) with a deterministic
1 ms zero-jitter retry overlay.

packages/llm/llm/README.md

@@ -54,6 +54,7 @@ Every product adapter sends application identity on provider HTTP requests. `att
 - `errorChain(value)` — renders a thrown value with its full `cause` chain and AggregateError members for diagnostic surfaces (UI notices, logger lines, durable `turn/end` messages), so transport wrappers like undici's `TypeError: fetch failed` surface the underlying `ECONNREFUSED`/DNS/TLS detail instead of masking it. Rendering only — route on `code`, never by parsing the result.
 - `CONTEXT_WINDOW_EXCEEDED_CODE` — the provider-neutral code both DeepSeek adapters use when a request exceeds the model context window, regardless of thrown-HTTP versus in-band finish delivery. `isContextWindowExceededError(detail)` is their shared conservative classifier for OpenAI-compatible provider detail.
 - `QUOTA_EXCEEDED_CODE` — the non-transient provider-neutral code for exhausted account quota, balance, credits, budget, or usage limits. `isQuotaExceededError(detail)` keeps those failures distinct from request-rate limits.
+- `EMPTY_RESPONSE_CODE` — the provider-neutral code both adapters use for a degenerate provider completion: a terminal `stop` that carried no content blocks at all. Classified as an error finish (not a successful empty message) because the attempt produced nothing durable; `dsh-llm-retry` retries it by default.
 
 ### Real adapters
 

packages/llm/llm/src/error.ts

@@ -27,6 +27,17 @@ export const CONTEXT_WINDOW_EXCEEDED_CODE = 'CONTEXT_WINDOW_EXCEEDED'
 /** Canonical provider-neutral code for an exhausted account quota or balance. */
 export const QUOTA_EXCEEDED_CODE = 'QUOTA'
 
+/**
+ * Canonical provider-neutral code for a response that completed normally but
+ * carried no content blocks at all. Providers occasionally emit a degenerate
+ * completion (a terminal stop with zero output); adapters classify it as this
+ * failure instead of yielding an empty assistant message, because an empty
+ * message silently ends the turn with nothing for the user or the loop to act
+ * on. The attempt produced nothing durable, so retry policy treats it as safe
+ * to repeat.
+ */
+export const EMPTY_RESPONSE_CODE = 'EMPTY_RESPONSE'
+
 /** Structured codes and plain phrases that explicitly name a context bound being exceeded. */
 const STRUCTURED_CONTEXT_OVERFLOW = new RegExp(
   String.raw`(?:^|[^a-z0-9])context[\s_-](?:length|window)[\s_-]`