fix(runner): narrow BaseException catch to Exception in tool execution

The tool execution path caught BaseException, which includes
KeyboardInterrupt, SystemExit, MemoryError, and GeneratorExit.
These should never be caught and converted into conversational
error messages. CancelledError is already handled separately.

Change except BaseException to except Exception so fatal signals
propagate instead of being swallowed.

Adds parametrized regression test for KeyboardInterrupt and
SystemExit propagation.

Fixes #4788

nanobot/agent/runner.py

@@ -1174,7 +1174,7 @@ class AgentRunner:
                 result = await spec.tools.execute(tool_call.name, params)
         except asyncio.CancelledError:
             raise
-        except BaseException as exc:
+        except Exception as exc:
             await hook.on_execute_tool_error(context, tool_call, tool, params, exc)
             event = {
                 "name": tool_call.name,