feat(llm-deepseek): support low reasoning effort

examples/acp-agent/tests/fs-diff-bound.cordis.snapshot.yml

@@ -41,6 +41,6 @@
                     - id: deepseek-v4-flash
                       contextWindow: 1000000
                       defaultMaxTokens: 256000
-                      reasoningEfforts: ['off', 'high', 'max']
+                      reasoningEfforts: ['off', 'low', 'high', 'max']
                       defaultReasoningEffort: max
                     - id: deepseek-v4-pro

examples/headless-agent/tests/fixtures/deepseek-defaults.cordis.yml

@@ -6,7 +6,8 @@
       - id: llm-deepseek
         config:
           baseURL: !!js process.env.DSH_SNAPSHOT_BASE_URL
-          thinking: disabled
+          thinking: enabled
+          reasoningEffort: low
           streamIdleTimeoutMs: 150
       - id: agent-spine
         config:

examples/headless-agent/tests/headless.snapshot.ts

@@ -541,6 +541,7 @@ describe('headless stream-json snapshots', () => {
       expect(result.stderr).toBe('')
       expect(server.requests).toHaveLength(1)
       expect(server.requests[0]?.max_tokens).toBe(256_000)
+      expect(server.requests[0]?.reasoning_effort).toBe('low')
       const header = (parseJsonl(result.stdout)
         .map(record => record.event)
         .find((event): event is JsonObject => (
@@ -555,7 +556,7 @@ describe('headless stream-json snapshots', () => {
           "maxTokens": 256000,
           "model": "deepseek-v4-flash",
           "provider": "deepseek-official",
-          "reasoningEffort": "off",
+          "reasoningEffort": "low",
         }
       `)
       expect(header?.adapterDefaults).toEqual({