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

optional-skills/migration/openclaw-migration/scripts/openclaw_to_hermes.py

@@ -1995,7 +1995,9 @@ class Migrator:
             if compaction.get("timeout"):
                 pass  # No direct mapping
             if compaction.get("model"):
-                compression["summary_model"] = compaction["model"]
+                aux = hermes_cfg.setdefault("auxiliary", {})
+                aux_comp = aux.setdefault("compression", {})
+                aux_comp["model"] = compaction["model"]
             hermes_cfg["compression"] = compression
             changes = True