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

run_agent.py

@@ -1212,7 +1212,6 @@ class AIAgent:
             _compression_cfg = {}
         compression_threshold = float(_compression_cfg.get("threshold", 0.50))
         compression_enabled = str(_compression_cfg.get("enabled", True)).lower() in ("true", "1", "yes")
-        compression_summary_model = _compression_cfg.get("summary_model") or None
         compression_target_ratio = float(_compression_cfg.get("target_ratio", 0.20))
         compression_protect_last = int(_compression_cfg.get("protect_last_n", 20))
 
@@ -1301,7 +1300,7 @@ class AIAgent:
                 protect_first_n=3,
                 protect_last_n=compression_protect_last,
                 summary_target_ratio=compression_target_ratio,
-                summary_model_override=compression_summary_model,
+                summary_model_override=None,
                 quiet_mode=self.quiet_mode,
                 base_url=self.base_url,
                 api_key=getattr(self, "api_key", ""),