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

website/docs/reference/environment-variables.md

@@ -14,6 +14,8 @@ All variables go in `~/.hermes/.env`. You can also set them with `hermes config
 |----------|-------------|
 | `OPENROUTER_API_KEY` | OpenRouter API key (recommended for flexibility) |
 | `OPENROUTER_BASE_URL` | Override the OpenRouter-compatible base URL |
+| `HERMES_OPENROUTER_CACHE` | Enable OpenRouter response caching (`1`/`true`/`yes`/`on`). Overrides `openrouter.response_cache` in config.yaml. See [Response Caching](https://openrouter.ai/docs/guides/features/response-caching). |
+| `HERMES_OPENROUTER_CACHE_TTL` | Cache TTL in seconds (1-86400). Overrides `openrouter.response_cache_ttl` in config.yaml. |
 | `NOUS_BASE_URL` | Override Nous Portal base URL (rarely needed; development/testing only) |
 | `NOUS_INFERENCE_BASE_URL` | Override Nous inference endpoint directly |
 | `AI_GATEWAY_API_KEY` | Vercel AI Gateway API key ([ai-gateway.vercel.sh](https://ai-gateway.vercel.sh)) |