Add top-level CLI error handling with defined exit codes and stderr discipline

痛点信号

• ## why this matters the first thing a new user with slightly-wrong data sees today is a python traceback ending in `valueerror` or `keyerror` — correct information, hostile presentation, and exit status that ci can't distinguish from a crash.
• ## current evidence - `cli.py::main` is `args.func(args)` with no error handling; `load_logged_decisions` and `offlineevaluator._validate_row` raise `valueerror`; a missing `--input` file raises `filenotfounderror` from `path.open`.
• wrap dispatch in `main()` with a small `try/except (valueerror, oserror)` mapping to message + `sys.exit(2)`; keep unexpected exceptions as tracebacks (genuine bugs should stay loud).
• ## acceptance criteria - missing input file → one-line stderr message, exit 2, no traceback.
• ## risks and tradeoffs over-catching can hide bugs — keep the caught set narrow and deliberate.