test: check agent stdin without heredoc pipe

[cbusillo]

Summary

• update the read-only agent stdin regression fixture to inspect fd 0 directly with python3 -c
• avoid the shell heredoc pipe that made the fixture report fifo on hosted Ubuntu even when the agent process stdin was detached

Validation

• cargo test -p code-core agent_tool::tests::read_only_agents_redirect_stdin_away_from_parent_pipe
• ./build-fast.sh

Refs #108
Refs #87