docs: comprehensive GitHub Copilot provider documentation
- Add dedicated GitHub Copilot section in configuration guide with:
- Auth options (OAuth device code, env vars, gh CLI)
- Token type table (supported vs unsupported)
- API routing explanation (GPT-5+ → Responses, others → Chat)
- Copilot ACP setup instructions
- Environment variable reference
- Add all Copilot env vars to environment-variables.md:
COPILOT_GITHUB_TOKEN, HERMES_COPILOT_ACP_COMMAND, etc.
- Add copilot-acp to --provider list in cli-commands.md
- Docs build verified
04101bc59e