fix(agent): resolve context_length for plugin context engines

Plugin context engines loaded via load_context_engine() were never
given context_length, causing the CLI status bar to show "ctx --"
with an empty progress bar. Call update_model() immediately after
loading the plugin engine, mirroring what switch_model() already does.

Fixes NousResearch/hermes-agent#9071

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

run_agent.py

@@ -1331,6 +1331,22 @@ class AIAgent:
 
         if _selected_engine is not None:
             self.context_compressor = _selected_engine
+            # Resolve context_length for plugin engines — mirrors switch_model() path
+            from agent.model_metadata import get_model_context_length
+            _plugin_ctx_len = get_model_context_length(
+                self.model,
+                base_url=self.base_url,
+                api_key=getattr(self, "api_key", ""),
+                config_context_length=_config_context_length,
+                provider=self.provider,
+            )
+            self.context_compressor.update_model(
+                model=self.model,
+                context_length=_plugin_ctx_len,
+                base_url=self.base_url,
+                api_key=getattr(self, "api_key", ""),
+                provider=self.provider,
+            )
             if not self.quiet_mode:
                 logger.info("Using context engine: %s", _selected_engine.name)
         else: