docs(guides): add OrcaRouter as an organisational LLM gateway in BYO-LLM guide - #827

This lets ArcKit users point Claude Code at OrcaRouter as a first-class organisational LLM gateway instead of treating it as an anonymous custom base URL. Teams on the BYO-LLM route get the same two-variable setup they already use for LiteLLM, Portkey and TrueFoundry, plus model discovery that surfaces OrcaRouter's provider/model catalogue (for example orcarouter/fusion-flash) straight into the /model picker.

OrcaRouter is an OpenAI-compatible AI gateway built for both models and agents. Like OpenRouter, it exposes a provider/model namespace across many models — but it also combines adaptive routing, automatic failover, zero-markup inference, observability, guardrails, and agent-tool governance behind the same endpoint. Adding orcarouter to Route 3 means this project's users can use that stack directly, without treating OrcaRouter as an anonymous custom base URL. 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.

The change mirrors the existing Route 3 entries: a named gateway bullet plus a config block using the same ANTHROPIC_BASE_URL / ANTHROPIC_AUTH_TOKEN variables, with the key prefix documented so the two-variable setup is copy-paste ready. The guide copy under plugins/arckit-claude/docs/guides/ was synced via python scripts/check-guide-parity.py --sync; --check passes (234 shared guides identical) and markdownlint-cli2 reports 0 errors on the three changed files. The CHANGELOG.md entry follows the existing Unreleased convention. Live connectivity to the documented base URL was verified with a real key before opening this PR.

Discord: discord.gg/YEubt8enRA · X: https://x.com/OrcaRouter

I'm an engineer on the OrcaRouter team.