fix: remove legacy compression.summary_* config and env var fallbacks (#8992)

Remove the backward-compat code paths that read compression provider/model
settings from legacy config keys and env vars, which caused silent failures
when auto-detection resolved to incompatible backends.

What changed:
- Remove compression.summary_model, summary_provider, summary_base_url from
  DEFAULT_CONFIG and cli.py defaults
- Remove backward-compat block in _resolve_task_provider_model() that read
  from the legacy compression section
- Remove _get_auxiliary_provider() and _get_auxiliary_env_override() helper
  functions (AUXILIARY_*/CONTEXT_* env var readers)
- Remove env var fallback chain for per-task overrides
- Update hermes config show to read from auxiliary.compression
- Add config migration (v16→17) that moves non-empty legacy values to
  auxiliary.compression and strips the old keys
- Update example config and openclaw migration script
- Remove/update tests for deleted code paths

Compression model/provider is now configured exclusively via:
  auxiliary.compression.provider / auxiliary.compression.model

Closes #8923

tests/tools/test_vision_tools.py

@@ -463,8 +463,6 @@ class TestVisionRequirements:
         monkeypatch.delenv("OPENROUTER_API_KEY", raising=False)
         monkeypatch.delenv("OPENAI_BASE_URL", raising=False)
         monkeypatch.delenv("OPENAI_API_KEY", raising=False)
-        monkeypatch.delenv("AUXILIARY_VISION_PROVIDER", raising=False)
-        monkeypatch.delenv("CONTEXT_VISION_PROVIDER", raising=False)
 
         assert check_vision_requirements() is True