Refactor pyconvert rule scoping

[cjdoris]

Summary

• replace pyconvert rule priorities with scoped rules and strict subclass ordering
• update built-in, wrapper, numpy, ctypes, and pandas conversions to use the new API
• refresh documentation, changelog, and tests to cover scoped rule behavior

Testing

• julia --project -e 'using PythonCall; println(pyconvert(Bool, pybuiltins.True))'
• julia --project -e 'using PythonCall; println(pyconvert(String, pystr("abc")))'

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Codex Task