Implement cleanup for stale multishell symlinks

[saikumarvasa100-hash]

This fix addresses issue #1478 where tsx watch and similar tools that spawn many Node.js processes were causing an excessive number of fnm_multishells symlinks to accumulate.

The cleanup_stale_multishells function now removes symlinks whose targets no longer exist, preventing the buildup of stale symlinks that was causing Task Manager freezes and preventing creation of new terminals.

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Latest commit: 2865fb2fe54705a3efbae084204ffd96f38bfcfd

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