document %cd ~ expansion as the idiomatic way to change kernel directory

[ncoop57]

Summary

• Adds a note in skill.py explaining that %cd expands ~ and is the preferred way to change the kernel's directory over os.chdir + expanduser.
• Clarifies that fastai file tools (fastcore, exhash, rgapi, aidialog) also expand ~ themselves, so paths with ~ pass straight through.

Why

Helps users avoid the common os.chdir(os.path.expanduser('~/...')) pattern by pointing them to the cleaner %cd magic, which already handles ~ expansion natively.