feat(compression): add summary_base_url + move compression config to YAML-only

- Add summary_base_url config option to compression block for custom
  OpenAI-compatible endpoints (e.g. zai, DeepSeek, Ollama)
- Remove compression env var bridges from cli.py and gateway/run.py
  (CONTEXT_COMPRESSION_* env vars no longer set from config)
- Switch run_agent.py to read compression config directly from
  config.yaml instead of env vars
- Fix backwards-compat block in _resolve_task_provider_model to also
  fire when auxiliary.compression.provider is 'auto' (DEFAULT_CONFIG
  sets this, which was silently preventing the compression section's
  summary_* keys from being read)
- Add test for summary_base_url config-to-client flow
- Update docs to show compression as config.yaml-only

Closes #1591
Based on PR #1702 by @uzaylisak

agent/auxiliary_client.py

@@ -1248,12 +1248,16 @@ def _resolve_task_provider_model(
         cfg_base_url = str(task_config.get("base_url", "")).strip() or None
         cfg_api_key = str(task_config.get("api_key", "")).strip() or None
 
-        # Backwards compat: compression section has its own keys
-        if task == "compression" and not cfg_provider:
+        # Backwards compat: compression section has its own keys.
+        # The auxiliary.compression defaults to provider="auto", so treat
+        # both None and "auto" as "not explicitly configured".
+        if task == "compression" and (not cfg_provider or cfg_provider == "auto"):
             comp = config.get("compression", {}) if isinstance(config, dict) else {}
             if isinstance(comp, dict):
                 cfg_provider = comp.get("summary_provider", "").strip() or None
                 cfg_model = cfg_model or comp.get("summary_model", "").strip() or None
+                _sbu = comp.get("summary_base_url") or ""
+                cfg_base_url = cfg_base_url or _sbu.strip() or None
 
     env_model = _get_auxiliary_env_override(task, "MODEL") if task else None
     resolved_model = model or env_model or cfg_model