feat(llm): register OrcaRouter in the provider profile system

[XiaoHuo888-hue]

What

Registers OrcaRouter as a named provider in Sirchmunk's capability
negotiation, alongside the existing named providers (DeepSeek, MiniMax,
SiliconFlow, etc.).

OrcaRouter (https://www.orcarouter.ai) is an
OpenAI-compatible router exposing 200+ models (OpenAI, Anthropic, Google,
DeepSeek, ...) behind a single API key. Model ids use the router's own
provider/model format (e.g. openai/gpt-4o). It already worked through
the generic OpenAI-compatible path (LLM_BASE_URL), and this change makes
it first-class:

• src/sirchmunk/llm/openai_chat.py: added "orcarouter" to the
  _PROVIDERS capability profiles and ("orcarouter.ai", "orcarouter") to
  _URL_PATTERNS, so a base URL of https://api.orcarouter.ai/v1 is
  auto-detected and negotiated with the generic-safe profile
  (stream_options enabled, no thinking parameter).
• README.md / README_zh.md: listed OrcaRouter in the supported
  providers, added an env config example (LLM_BASE_URL, LLM_API_KEY,
  LLM_MODEL_NAME), and a "Using with OrcaRouter" Python SDK example.

Why

OrcaRouter users previously had to type the endpoint manually into the
generic OpenAI-compatible path. A named profile presets the endpoint,
correctly negotiates streaming options, and surfaces OrcaRouter in the
documented provider list — the same treatment the other named providers get.

How to Test

1. Live API (real OrcaRouter key): OpenAIChat(api_key="sk-orca-...", base_url="https://api.orcarouter.ai/v1", model="openai/gpt-4o").chat(...)
   returns "hello from orcarouter" (non-streaming and streaming, with usage).
2. Auto-detection: OpenAIChat._detect_provider("https://api.orcarouter.ai/v1")
   returns the orcarouter profile.
3. uvx --from ruff@0.8.4 ruff check src/sirchmunk/llm/openai_chat.py — clean.

Related Issue

None.

Checklist

• py_compile clean; provider registration + detection verified
• Live test with a real OrcaRouter key (non-stream + stream via
  OpenAIChat, both return usage)
• ruff (repo-pinned v0.8.4) clean on the changed file
• black — note: the changed file is not black-formatted at main
  (pre-existing, whole file), so the diff matches the file's existing
  alignment style rather than reformatting the file.

Disclosure: I'm an engineer on the OrcaRouter team.