feat: add stdin to run_skill_script

[micpapal]

Problem

run_skill_script has no way to feed stdin to the scripts it runs. sandbox.run_script()'s subprocess.run(...) call never passes a stdin=/input= argument, so a launched script inherits this process's own stdin — which never produces data or an EOF here. Any script that reads stdin (e.g. a --file - / "or on stdin" convention) blocks until timeout.

Found via a real failure: an agent calling concord.py record reply --file - to record an agent's reply. Every call timed out (exit_code: 124).

Fix

• sandbox.run_script() gains stdin: str = "", always passed as subprocess.run's input= — immediate EOF when empty, real content when given. Either way, nothing blocks waiting for stdin that will never arrive.
• run_skill_script's tool schema gains a matching optional "stdin" parameter, threaded through to the call above.

[jordanauge]

lgtm