core/cli: Click slash registry with awaitlet bridge

Register REPL slash commands as a Click Group (params, UsageError, help).
Dispatch via awaitlet.async_def so sync command bodies can awaitlet()
async memory/confirm work. Completer and /help derive from the registry.

tests/test_cli/test_repl_commands.py

@@ -6,7 +6,7 @@ import pytest
 import tomlkit
 
 from plyngent.agent import ChatAgent
-from plyngent.cli.repl import handle_slash
+from plyngent.cli.slash import handle_slash
 from plyngent.cli.state import ReplState
 from plyngent.config.models import DatabaseConfig, OpenAIProvider
 from plyngent.config.store import ConfigStore