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

hermes_cli/config.py

@@ -1082,7 +1082,6 @@ ENV_VARS_BY_VERSION: Dict[int, List[str]] = {
         "SLACK_BOT_TOKEN", "SLACK_APP_TOKEN", "SLACK_ALLOWED_USERS"],
     10: ["TAVILY_API_KEY"],
     11: ["TERMINAL_MODAL_MODE"],
-    17: ["GMI_API_KEY", "GMI_BASE_URL"],
 }
 
 # Required environment variables with metadata for migration prompts.