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-retry/src/index.ts

@@ -33,7 +33,7 @@ const DEFAULT_MAX_TRANSIENT_RETRIES = 2
 const DEFAULT_INITIAL_DELAY_MS = 500
 const DEFAULT_MAX_DELAY_MS = 10_000
 const DEFAULT_JITTER_RATIO = 0.1
-const DEFAULT_RETRYABLE_CODES = Object.freeze(['RATE_LIMIT', 'SERVER', 'TIMEOUT', 'TRANSPORT'])
+const DEFAULT_RETRYABLE_CODES = Object.freeze(['EMPTY_RESPONSE', 'RATE_LIMIT', 'SERVER', 'TIMEOUT', 'TRANSPORT'])
 
 /** Deployment-owned limits and classification for transient request recovery. */
 export interface Config {