Persist awsctx context across shells

[lox]

Summary

• persist the selected AWS profile/region to an XDG state file
• restore persisted context automatically when zsh/aws.zsh is sourced
• clear persisted context when awsctx --clear is used
• have awssh attempt restore before starting SSM sessions

Verification

• zsh -n zsh/aws.zsh
• manual smoke test for save/restore/clear with temporary XDG_STATE_HOME