Fix crash on variadic tool path arguments

[grfwings]

Fixes #738

Effect CLI currently requires an explicit options object for Argument.variadic; without it, the parsed pathParts value
becomes {} instead of an array. That made executor call ... crash before invoking the tool.

This updates the call command to parse path segments with:

  Args.string("tool-path-segment").pipe(Args.variadic({}))

and adds an explicit failure if the parser ever returns a non-array value.

Upstream issue: Effect-TS/effect#6228

Also, added .pi/ to .gitignore.

[RhysSullivan]

thanks!