feat(openrouter): add response caching support (#19132)

Enable OpenRouter's response caching feature (beta) via X-OpenRouter-Cache
headers. When enabled, identical API requests return cached responses for
free (zero billing), reducing both latency and cost.

Configuration via config.yaml:
  openrouter:
    response_cache: true       # default: on
    response_cache_ttl: 300    # 1-86400 seconds

Changes:
- Add openrouter config section to DEFAULT_CONFIG (response_cache + TTL)
- Add build_or_headers() in auxiliary_client.py that builds attribution
  headers plus optional cache headers based on config
- Replace inline _OR_HEADERS dicts with build_or_headers() at all 5 sites:
  run_agent.py __init__, _apply_client_headers_for_base_url(), and
  auxiliary_client.py _try_openrouter() + _to_async_client()
- Add _check_openrouter_cache_status() method to AIAgent that reads
  X-OpenRouter-Cache-Status from streaming response headers and logs
  HIT/MISS status
- Document in cli-config.yaml.example
- Add 28 tests (22 unit + 6 integration)

Ref: https://openrouter.ai/docs/guides/features/response-caching

run_agent.py

@@ -1258,6 +1258,10 @@ class AIAgent:
         # after each API call.  Accessed by /usage slash command.
         self._rate_limit_state: Optional["RateLimitState"] = None
 
+        # OpenRouter response cache hit counter — incremented when
+        # X-OpenRouter-Cache-Status: HIT is seen in streaming response headers.
+        self._or_cache_hits: int = 0
+
         # Centralized logging — agent.log (INFO+) and errors.log (WARNING+)
         # both live under ~/.hermes/logs/.  Idempotent, so gateway mode
         # (which creates a new AIAgent per message) won't duplicate handlers.
@@ -1421,11 +1425,8 @@ class AIAgent:
                     client_kwargs["args"] = self.acp_args
                 effective_base = base_url
                 if base_url_host_matches(effective_base, "openrouter.ai"):
-                    client_kwargs["default_headers"] = {
-                        "HTTP-Referer": "https://hermes-agent.nousresearch.com",
-                        "X-OpenRouter-Title": "Hermes Agent",
-                        "X-OpenRouter-Categories": "productivity,cli-agent",
-                    }
+                    from agent.auxiliary_client import build_or_headers
+                    client_kwargs["default_headers"] = build_or_headers()
                 elif base_url_host_matches(effective_base, "api.routermint.com"):
                     client_kwargs["default_headers"] = _routermint_headers()
                 elif base_url_host_matches(effective_base, "api.githubcopilot.com"):
@@ -4580,6 +4581,28 @@ class AIAgent:
         """Return the last captured RateLimitState, or None."""
         return self._rate_limit_state
 
+    def _check_openrouter_cache_status(self, http_response: Any) -> None:
+        """Read X-OpenRouter-Cache-Status from response headers and log it.
+
+        Increments ``_or_cache_hits`` on HIT so callers can report savings.
+        """
+        if http_response is None:
+            return
+        headers = getattr(http_response, "headers", None)
+        if not headers:
+            return
+        try:
+            status = headers.get("x-openrouter-cache-status")
+            if not status:
+                return
+            if status.upper() == "HIT":
+                self._or_cache_hits += 1
+                logger.info("OpenRouter response cache HIT (total: %d)", self._or_cache_hits)
+            else:
+                logger.debug("OpenRouter response cache %s", status.upper())
+        except Exception:
+            pass  # Never let header parsing break the agent loop
+
     def get_activity_summary(self) -> dict:
         """Return a snapshot of the agent's current activity for diagnostics.
 
@@ -6157,10 +6180,10 @@ class AIAgent:
         return True
 
     def _apply_client_headers_for_base_url(self, base_url: str) -> None:
-        from agent.auxiliary_client import _AI_GATEWAY_HEADERS, _OR_HEADERS
+        from agent.auxiliary_client import _AI_GATEWAY_HEADERS, build_or_headers
 
         if base_url_host_matches(base_url, "openrouter.ai"):
-            self._client_kwargs["default_headers"] = dict(_OR_HEADERS)
+            self._client_kwargs["default_headers"] = build_or_headers()
         elif base_url_host_matches(base_url, "ai-gateway.vercel.sh"):
             self._client_kwargs["default_headers"] = dict(_AI_GATEWAY_HEADERS)
         elif base_url_host_matches(base_url, "api.routermint.com"):
@@ -6780,6 +6803,9 @@ class AIAgent:
             # response via .response before any chunks are consumed.
             self._capture_rate_limits(getattr(stream, "response", None))
 
+            # Log OpenRouter response cache status when present.
+            self._check_openrouter_cache_status(getattr(stream, "response", None))
+
             content_parts: list = []
             tool_calls_acc: dict = {}
             tool_gen_notified: set = set()