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

website/docs/reference/environment-variables.md

@@ -18,6 +18,13 @@ All variables go in `~/.hermes/.env`. You can also set them with `hermes config
 | `AI_GATEWAY_BASE_URL` | Override AI Gateway base URL (default: `https://ai-gateway.vercel.sh/v1`) |
 | `OPENAI_API_KEY` | API key for custom OpenAI-compatible endpoints (used with `OPENAI_BASE_URL`) |
 | `OPENAI_BASE_URL` | Base URL for custom endpoint (VLLM, SGLang, etc.) |
+| `COPILOT_GITHUB_TOKEN` | GitHub token for Copilot API — first priority (OAuth `gho_*` or fine-grained PAT `github_pat_*`; classic PATs `ghp_*` are **not supported**) |
+| `GH_TOKEN` | GitHub token — second priority for Copilot (also used by `gh` CLI) |
+| `GITHUB_TOKEN` | GitHub token — third priority for Copilot |
+| `HERMES_COPILOT_ACP_COMMAND` | Override Copilot ACP CLI binary path (default: `copilot`) |
+| `COPILOT_CLI_PATH` | Alias for `HERMES_COPILOT_ACP_COMMAND` |
+| `HERMES_COPILOT_ACP_ARGS` | Override Copilot ACP arguments (default: `--acp --stdio`) |
+| `COPILOT_ACP_BASE_URL` | Override Copilot ACP base URL |
 | `GLM_API_KEY` | z.ai / ZhipuAI GLM API key ([z.ai](https://z.ai)) |
 | `ZAI_API_KEY` | Alias for `GLM_API_KEY` |
 | `Z_AI_API_KEY` | Alias for `GLM_API_KEY` |