feat: add OrcaRouter as an LLM provider

[XiaoHuo888-hue]

Summary

This registers OrcaRouter the same way the existing OpenRouter and Atlas Cloud providers are wired, so the config reference and docs stay consistent. OrcaRouter is an OpenAI-compatible gateway: one ORCAROUTER_API_KEY (keys start with sk-orca-) unlocks 150+ models from OpenAI, Anthropic, Google, DeepSeek, Qwen, MiniMax and xAI behind a single https://api.orcarouter.ai/v1 endpoint. It also runs gateway-level, zero-trust security for AI agents on the same endpoint - screening every prompt/response and governing every tool call on a default-deny basis, with no application code changes.

I'm an engineer on the OrcaRouter team.

What this changes

• src/standard/llm_client.py: registers the orcarouter provider in PROVIDER_DEFAULTS (base URL https://api.orcarouter.ai/v1, default model deepseek/deepseek-chat, config key orcarouter_api_key), with an ORCAROUTER_API_KEY env fallback
• config/config.example.toml: adds orcarouter_api_key and updates the provider comment
• README.md / README-zh.md: adds an OrcaRouter config block alongside the existing providers
• docs/configuration.md: adds an OrcaRouter section, provider-defaults table row, and env-var row
• docs/api-reference.md / docs/pipeline.md: documents the new provider and default model
• tests/test_llm_client.py: adds test_orcarouter_provider and test_orcarouter_env_key

How to use it

1. Get an API key at https://www.orcarouter.ai/console (keys start with sk-orca-).
2. Set orcarouter_api_key in config/config.toml, or ORCAROUTER_API_KEY as an env var.
3. Set [llm] provider = "orcarouter" (the default model is deepseek/deepseek-chat; any OrcaRouter model slug works).

Validation

• Unit tests: pytest tests/ -q -> 28 passed (includes the 2 new OrcaRouter tests)
• python -m py_compile on the changed Python files + git diff --check clean
• Live API with a real key through the repo's own resolve_llm_config + chat_completions path: default model 200, orcarouter/auto 200, invalid key 401
• Confirmed deepseek/deepseek-chat is present in the live OrcaRouter model catalog

README/docs updates are limited to existing technical provider configuration sections; no sponsor/logo/credits/partner content was added.