Make container command timeout configurable

[eshulman2]

Summary

• add a positive CONTAINER_COMMAND_TIMEOUT setting with a 600-second default
• pass it into task containers and apply it to implementation commands, reviewer commands, and fallback tests
• show the effective timeout in the container system prompt
• document the setting and validate invalid values safely

Testing

• Ruff passed for all changed Python files
• 71 focused tests passed