feat: wire context engine plugin slot into agent and plugin system

- PluginContext.register_context_engine() lets plugins replace the
  built-in ContextCompressor with a custom ContextEngine implementation
- PluginManager stores the registered engine; only one allowed
- run_agent.py checks for a plugin engine at init before falling back
  to the default ContextCompressor
- reset_session_state() now calls engine.on_session_reset() instead of
  poking internal attributes directly
- ContextCompressor.on_session_reset() handles its own internals
  (_context_probed, _previous_summary, etc.)
- 19 new tests covering ABC contract, defaults, plugin slot registration,
  rejection of duplicates/non-engines, and compressor reset behavior
- All 34 existing compressor tests pass unchanged

agent/context_compressor.py

@@ -66,6 +66,13 @@ class ContextCompressor(ContextEngine):
     def name(self) -> str:
         return "compressor"
 
+    def on_session_reset(self) -> None:
+        """Reset all per-session state for /new or /reset."""
+        super().on_session_reset()
+        self._context_probed = False
+        self._context_probe_persistable = False
+        self._previous_summary = None
+
     def __init__(
         self,
         model: str,