Document Base-native Bash script shebang pattern

[codeforester]

Summary

Document how to write Bash scripts that run through Base using the #!/usr/bin/env basectl shebang and the Base standard shell library.

The behavior is implemented and partially documented in docs/execution-model.md, STANDARDS.md, and lib/bash/std/README.md, but the user-facing guidance is scattered.

Acceptance Criteria

• FAQ includes a clear question for writing a Base-native Bash script.
• Docs explain that shebang-based Base scripts define main and do not call main "$@" themselves.
• Docs explain that basectl loads base_init.sh, stdlib helpers, Base runtime variables, and then calls main.
• Docs contrast Base-run scripts with standalone Bash scripts that source lib_std.sh directly.
• Docs link to the execution model, standards, and stdlib README for deeper detail.

Notes

This is documentation-only. No runtime behavior should change.