docs: add Xiaomi MiMo to all provider docs + fix MiMo-V2-Flash ctx len

- environment-variables.md: XIAOMI_API_KEY, XIAOMI_BASE_URL, provider list
- cli-commands.md: --provider choices
- integrations/providers.md: provider table, Chinese providers section,
  config example, base URL list, choosing table, fallback providers list
- fallback-providers.md: supported providers table, auto-detection chain
- Fix XiaomiMiMo/MiMo-V2-Flash context length 32768 → 256000 (OpenRouter entry)

website/docs/reference/cli-commands.md

@@ -76,7 +76,7 @@ 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-acp`, `copilot`, `anthropic`, `huggingface`, `zai`, `kimi-coding`, `minimax`, `minimax-cn`, `deepseek`, `ai-gateway`, `opencode-zen`, `opencode-go`, `kilocode`, `alibaba`. |
+| `--provider <provider>` | Force a provider: `auto`, `openrouter`, `nous`, `openai-codex`, `copilot-acp`, `copilot`, `anthropic`, `huggingface`, `zai`, `kimi-coding`, `minimax`, `minimax-cn`, `deepseek`, `ai-gateway`, `opencode-zen`, `opencode-go`, `kilocode`, `xiaomi`, `alibaba`. |
 | `-s`, `--skills <name>` | Preload one or more skills for the session (can be repeated or comma-separated). |
 | `-v`, `--verbose` | Verbose output. |
 | `-Q`, `--quiet` | Programmatic mode: suppress banner/spinner/tool previews. |