fix(security): harden ExecTool defaults against command injection (#506) (#507)

* fix(security): harden ExecTool defaults against command injection (#506)

- Default restrict_to_workspace to True (was False), confining commands
  to the working directory unless explicitly opted out
- Add deny patterns for network exfiltration (curl, wget, nc, ssh, scp,
  rsync, ftp, socat)
- Add deny patterns for script interpreter exfiltration (python -c,
  perl -e, ruby -e)
- Add deny patterns for sensitive file access (/etc/passwd, /etc/shadow,
  /etc/sudoers) and privilege escalation (useradd, usermod, crontab,
  chmod world-writable)
- Add comprehensive test suite (32 tests) covering deny patterns,
  workspace restriction, and command execution

* fix(tests): use pytest.mark.asyncio for Python 3.14 compatibility

asyncio.get_event_loop() raises RuntimeError in Python 3.14 when
no event loop is running. Switch to pytest-asyncio's async test
decorator which is already a project dependency.