Traceback on exit in fast-agent go (--watch/interactive)

[iqdoctor]

When running fast-agent go in interactive mode (e.g. --card agents --watch), typing exit raises PromptExitError and the CLI prints a full traceback instead of a clean exit.

Repro:

1. uv run fast-agent go --card agents --watch
2. Type exit

Observed stack trace (trimmed):

PromptExitError: User requested to exit fast-agent session
  File ".../fast_agent/cli/commands/go.py", line 584, in go
    run_async_agent(...)
  File ".../fast_agent/core/fastagent.py", line 1251, in run
    yield wrapper
  File ".../fast_agent/ui/enhanced_prompt.py", line 1587, in handle_special_commands
    raise PromptExitError("User requested to exit fast-agent session")

Expected: clean exit without traceback.

I have a fix in my fork that catches SystemExit in the CLI runner and suppresses the traceback for exit code 0.