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/cli-commands.md

@@ -66,7 +66,7 @@ Common options:
 | `-q`, `--query "..."` | One-shot, non-interactive prompt. |
 | `-m`, `--model <model>` | Override the model for this run. |
 | `-t`, `--toolsets <csv>` | Enable a comma-separated set of toolsets. |
-| `--provider <provider>` | Force a provider: `auto`, `openrouter`, `nous`, `openai-codex`, `copilot`, `anthropic`, `zai`, `kimi-coding`, `minimax`, `minimax-cn`. |
+| `--provider <provider>` | Force a provider: `auto`, `openrouter`, `nous`, `openai-codex`, `copilot`, `copilot-acp`, `anthropic`, `zai`, `kimi-coding`, `minimax`, `minimax-cn`. |
 | `-v`, `--verbose` | Verbose output. |
 | `-Q`, `--quiet` | Programmatic mode: suppress banner/spinner/tool previews. |
 | `--resume <session>` / `--continue [name]` | Resume a session directly from `chat`. |