fix: guard empty plugin args

[operagxoksana]

No description provided.

[cursor]

PR Summary

Improves robustness of external plugin execution by validating args before use.

• In execute_external_subcommand, replace first().expect(...) with an error return (anyhow!("no subcommand provided")) when no subcommand is supplied
• All other behavior for locating and invoking forc-<cmd> remains unchanged

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