refactor(agent): exhaust recovery and compaction decisions

Dispatch RequestErrorDecision and CompactionTrigger through explicit discriminant switches. End each closed union with assertNever so new variants fail compilation instead of silently inheriting fail or pressure behavior.

This preserves the current retry, fail, pressure, and overflow semantics while aligning the new recovery seams with the repository closed-union contract.

packages/core/agent-loop/src/loop.ts

@@ -8,7 +8,7 @@
 import type { Context } from 'cordis'
 import type { ContentBlock, FinishReason, GenerateOptions, LlmCallConfig, Message } from '@deepseek-ai/dsh-llm'
 import { isDeepStrictEqual } from 'node:util'
-import { BlockAssembler, HarnessError, deepFreeze, isLlmAdapterFailure } from '@deepseek-ai/dsh-llm'
+import { BlockAssembler, HarnessError, assertNever, deepFreeze, isLlmAdapterFailure } from '@deepseek-ai/dsh-llm'
 import { agentEvents, assembleContextFor } from '@deepseek-ai/dsh-agent'
 import type { AgentEventDispatch, ContinuationDecision, HookContext, PromptDecision, RequestError, RequestErrorDecision } from '@deepseek-ai/dsh-agent'
 import { canonicalHeader } from '@deepseek-ai/dsh-session'
@@ -392,11 +392,17 @@ async function runTurn(
             : { kind: 'aborted', reason: String(abort.signal.reason) }
           break
         }
-        if (recoveryDecision.action === 'retry') {
-          requestRetryAttempt += 1
-          continue
+        switch (recoveryDecision.action) {
+          case 'retry':
+            requestRetryAttempt += 1
+            continue
+          case 'fail':
+            failTurn(stepOutcome.requestError)
+            break
+          /* v8 ignore next -- closed-union exhaustiveness guard */
+          default:
+            assertNever(recoveryDecision, 'agent request-error decision')
         }
-        failTurn(stepOutcome.requestError)
         break
       }