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

cli-config.yaml.example

@@ -309,15 +309,8 @@ compression:
   # compression of older turns.
   protect_last_n: 20
 
-  # Model to use for generating summaries (fast/cheap recommended)
-  # This model compresses the middle turns into a concise summary.
-  # IMPORTANT: it receives the full middle section of the conversation, so it
-  # MUST support a context length at least as large as your main model's.
-  summary_model: "google/gemini-3-flash-preview"
-  
-  # Provider for the summary model (default: "auto")
-  # Options: "auto", "openrouter", "nous", "main"
-  # summary_provider: "auto"
+  # To pin a specific model/provider for compression summaries, use the
+  # auxiliary section below (auxiliary.compression.provider / model).
 
 # =============================================================================
 # Auxiliary Models (Advanced — Experimental)