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/cli/test_cli_save_config_value.py

@@ -51,10 +51,10 @@ class TestSaveConfigValueAtomic:
     def test_creates_nested_keys(self, config_env):
         """Dot-separated paths create intermediate dicts as needed."""
         from cli import save_config_value
-        save_config_value("compression.summary_model", "google/gemini-3-flash-preview")
+        save_config_value("auxiliary.compression.model", "google/gemini-3-flash-preview")
 
         result = yaml.safe_load(config_env.read_text())
-        assert result["compression"]["summary_model"] == "google/gemini-3-flash-preview"
+        assert result["auxiliary"]["compression"]["model"] == "google/gemini-3-flash-preview"
 
     def test_overwrites_existing_value(self, config_env):
         """Updating an existing key replaces the value."""