fix: add __main__ guards to eth2-quickstart, mubu, siyuan and zotero CLIs (#440)

These four CLI modules define their entry point but never invoke it under an
`if __name__ == "__main__":` guard, so `python -m cli_anything.<pkg>.<pkg>_cli`
exits 0 without producing any output. 63 of the 68 CLI modules in the repo
already carry the guard; this brings the remaining ones in line.

Each guard calls the same callable the console script points at, so behaviour
matches an installed run:

- eth2-quickstart -> `main()`
- mubu            -> `raise SystemExit(entrypoint())`
- siyuan          -> `cli()`
- zotero          -> `raise SystemExit(entrypoint())`

`entrypoint()` returns an exit code in mubu and zotero, which `SystemExit`
propagates the same way setuptools' console-script wrapper does.

Refs #436


Claude-Session: https://claude.ai/code/session_01ChPxA9ksnNqr83nLbfMhwu

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>