fix: robust context engine interface — config selection, plugin discovery, ABC completeness

Follow-up fixes for the context engine plugin slot (PR #5700):

- Enhance ContextEngine ABC: add threshold_percent, protect_first_n,
  protect_last_n as class attributes; complete update_model() default
  with threshold recalculation; clarify on_session_end() lifecycle docs
- Add ContextCompressor.update_model() override for model/provider/
  base_url/api_key updates
- Replace all direct compressor internal access in run_agent.py with
  ABC interface: switch_model(), fallback restore, context probing
  all use update_model() now; _context_probed guarded with getattr/
  hasattr for plugin engine compatibility
- Create plugins/context_engine/ directory with discovery module
  (mirrors plugins/memory/ pattern) — discover_context_engines(),
  load_context_engine()
- Add context.engine config key to DEFAULT_CONFIG (default: compressor)
- Config-driven engine selection in run_agent.__init__: checks config,
  then plugins/context_engine/<name>/, then general plugin system,
  falls back to built-in ContextCompressor
- Wire on_session_end() in shutdown_memory_provider() at real session
  boundaries (CLI exit, /reset, gateway expiry)

agent/context_compressor.py

@@ -73,6 +73,22 @@ class ContextCompressor(ContextEngine):
         self._context_probe_persistable = False
         self._previous_summary = None
 
+    def update_model(
+        self,
+        model: str,
+        context_length: int,
+        base_url: str = "",
+        api_key: str = "",
+        provider: str = "",
+    ) -> None:
+        """Update model info after a model switch or fallback activation."""
+        self.model = model
+        self.base_url = base_url
+        self.api_key = api_key
+        self.provider = provider
+        self.context_length = context_length
+        self.threshold_tokens = int(context_length * self.threshold_percent)
+
     def __init__(
         self,
         model: str,