fix(providers/gmi): post-salvage review fixes

- config.py: remove dead ENV_VARS_BY_VERSION[17] entry (current _config_version
  is 22, so all users are past version 17 and would never be prompted for
  GMI_API_KEY on upgrade — consistent with how arcee was added)
- auxiliary_client.py: use google/gemini-3.1-flash-lite-preview as GMI aux
  model instead of anthropic/claude-opus-4.6 (matches cheap fast-model pattern
  used by all other providers: zai→glm-4.5-flash, kimi→kimi-k2-turbo-preview,
  stepfun→step-3.5-flash, kilocode→google/gemini-3-flash-preview)
- test_gmi_provider.py: fix malformed write_text() call in doctor test
  (was: write_text("GMI_API_KEY=*** encoding="utf-8") → missing closing quote,
  wrote literal string 'GMI_API_KEY=*** encoding=' to .env file)
- test_gmi_provider.py + test_auxiliary_client.py: update aux model assertions
  to match new cheaper default
- docs/integrations/providers.md: add 'gmi' to inline 'Supported providers'
  fallback list (was only in the table, not the inline list at line ~1181)
- docs/reference/cli-commands.md: add 'gmi' to --provider choices list

website/docs/reference/cli-commands.md

@@ -85,7 +85,7 @@ 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`, `gemini`, `google-gemini-cli`, `huggingface`, `zai`, `kimi-coding`, `kimi-coding-cn`, `minimax`, `minimax-cn`, `kilocode`, `xiaomi`, `arcee`, `alibaba`, `deepseek`, `nvidia`, `ollama-cloud`, `xai` (alias `grok`), `qwen-oauth`, `bedrock`, `opencode-zen`, `opencode-go`, `ai-gateway`, `azure-foundry`. |
+| `--provider <provider>` | Force a provider: `auto`, `openrouter`, `nous`, `openai-codex`, `copilot-acp`, `copilot`, `anthropic`, `gemini`, `google-gemini-cli`, `huggingface`, `zai`, `kimi-coding`, `kimi-coding-cn`, `minimax`, `minimax-cn`, `kilocode`, `xiaomi`, `arcee`, `gmi`, `alibaba`, `deepseek`, `nvidia`, `ollama-cloud`, `xai` (alias `grok`), `qwen-oauth`, `bedrock`, `opencode-zen`, `opencode-go`, `ai-gateway`, `azure-foundry`. |
 | `-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. |