feat(openrouter): add opt-in response caching

Adds opt-in OpenRouter response caching params, preserves alias precedence across config scopes, and documents the behavior.\n\nVerification:\n- pnpm test:serial src/agents/pi-embedded-runner/proxy-stream-wrappers.test.ts src/agents/pi-embedded-runner-extraparams-openrouter.test.ts -- --reporter=verbose\n- pnpm exec oxfmt --check --threads=1 src/agents/pi-embedded-runner/proxy-stream-wrappers.ts src/plugin-sdk/provider-stream.ts src/agents/pi-embedded-runner/proxy-stream-wrappers.test.ts src/agents/pi-embedded-runner-extraparams-openrouter.test.ts docs/providers/openrouter.md CHANGELOG.md\n- git diff --check\n- Testbox tbx_01kqr4dakpsk9rswz9pem49nz0: pnpm check:changed (https://github.com/openclaw/openclaw/actions/runs/25294515012)

docs/providers/openrouter.md

@@ -153,6 +153,39 @@ does **not** inject those OpenRouter-specific headers or Anthropic cache markers
 ## Advanced configuration
 
 <AccordionGroup>
+  <Accordion title="Response caching">
+    OpenRouter response caching is opt-in. Enable it per OpenRouter model with
+    model params:
+
+    ```json5
+    {
+      agents: {
+        defaults: {
+          models: {
+            "openrouter/auto": {
+              params: {
+                responseCache: true,
+                responseCacheTtlSeconds: 300,
+              },
+            },
+          },
+        },
+      },
+    }
+    ```
+
+    OpenClaw sends `X-OpenRouter-Cache: true` and, when configured,
+    `X-OpenRouter-Cache-TTL`. `responseCacheClear: true` forces a refresh for
+    the current request and stores the replacement response. Snake_case aliases
+    (`response_cache`, `response_cache_ttl_seconds`, and
+    `response_cache_clear`) are also accepted.
+
+    This is separate from provider prompt caching and from OpenRouter's
+    Anthropic `cache_control` markers. It is only applied on verified
+    `openrouter.ai` routes, not custom proxy base URLs.
+
+  </Accordion>
+
   <Accordion title="Anthropic cache markers">
     On verified OpenRouter routes, Anthropic model refs keep the
     OpenRouter-specific Anthropic `cache_control` markers that OpenClaw uses for