feat(cli): rewrite input layer with prompt_toolkit and polish UI

- Replaces fragile input() hacks with robust prompt_toolkit.PromptSession
- Native support for multiline paste, history, and clean display
- Restores animated spinner in _thinking_ctx (now safe)
- Replaces boxed Panel with clean header for easier copying
- Adds prompt-toolkit dependency
- Adds new unit tests for input layer

pyproject.toml

@@ -37,6 +37,7 @@ dependencies = [
     "slack-sdk>=3.26.0",
     "qq-botpy>=1.0.0",
     "python-socks[asyncio]>=2.4.0",
+    "prompt-toolkit>=3.0.0",
 ]
 
 [project.optional-dependencies]