feat: add extra_headers support for custom providers

[jneeee]

Summary

 Custom providers in hermes-agent can now specify custom HTTP headers for authentication, routing, or client identification.

 ## Changes

 ### Core Implementation

 - **`hermes_cli/runtime_provider.py`**:
   - Added `extra_headers` reading in `_get_named_custom_provider()` (line 170-172)
   - Added `extra_headers` passing through in `_resolve_named_custom_runtime()` (line 211)

 - **`run_agent.py`**:
   - Added `extra_headers: Dict[str, str] = None` parameter to `AIAgent.__init__()` (line 432)
   - Stored as instance variable `self.extra_headers` (line 500)
   - Applied to `client_kwargs["default_headers"]` when provider is "custom" (lines 753-755)

 - **`gateway/run.py`**:
   - Added `extra_headers` to `_resolve_runtime_agent_kwargs()` return dict (line 257)

 ### Documentation

 - **`website/docs/user-guide/configuration.md`**:
   - Added `extra_headers` example in custom provider YAML config
   - Added "Custom HTTP Headers" subsection explaining usage and common headers

 ## Config Format

 Users can add custom providers with `extra_headers` in `config.yaml`:

 ```yaml
 custom_providers:
   - name: "my-custom"
     base_url: "https://my-inference-server.com/v1"
     api_key: "${MY_API_KEY}"
     api_mode: "chat_completions"
     extra_headers:
       X-Custom-Auth: "my-token"
       X-Client-Name: "hermes-agent"
 ```

 ## Test Plan

 - [x] `test_named_custom_provider_with_extra_headers` - verifies extra_headers are passed through runtime resolution
 - [x] `test_named_custom_provider_extra_headers_not_set_when_missing` - verifies behavior when extra_headers is absent
 - [x] `test_extra_headers_passed_to_openai_client_for_custom_provider` - verifies headers reach the OpenAI client
 - [x] `test_extra_headers_not_set_for_non_custom_provider` - verifies OpenRouter headers take precedence
 - [x] `test_no_default_headers_when_no_extra_headers_and_custom_provider` - verifies no headers set when none provided

 ## Files Changed

 ```
 gateway/run.py                            |  1 +
 hermes_cli/runtime_provider.py            |  4 ++
 run_agent.py                              |  7 +++
 tests/test_run_agent.py                   | 95 +++++++++++++++++++++++++++++++
 tests/test_runtime_provider_resolution.py | 46 +++++++++++++++++++
 website/docs/user-guide/configuration.md   | 30 +++++++++++++
 6 files changed, 183 insertions(+)
 ```