Address the complexity of main()

[udf2457]

As a controversial disclosure, this PR is LLM assisted. I am not a vibecoder, but "clean up main()" seemed to me to be a focused enough task to try out, and given #43 has been open since March ...

This is what the LLM spat out and it seems like a reasonable attempt.

1. Struct-based flags – all flags live in an opts struct, eliminating the sea of local pflag variables in main.
2. Centralized validation – required flags per command and USS flag rules are checked in one place (or single helpers), not duplicated inside case blocks.
3. defer signer.Close() – the signer is closed automatically when runGetKey or runSign return, removing the manual signer.Close() calls before every os.Exit.
4. Default handling – the .sig file default is set in one helper (setDefaults), not twice.
5. main() is purely dispatch – parse → validate → run → exit. All business logic lives in runGetKey, runSign, and runVerify.