Clarify Codex App worktree compatibility in using-git-worktrees

[q107580018]

Summary

• prefer Codex App-managed worktrees when App compatibility matters
• treat manual git worktree creation as a fallback path rather than the default
• update Python setup guidance to prefer uv and clarify manual worktree location rules

Verification

• ran git diff --check
• reviewed the resulting staged diff for skills/using-git-worktrees/SKILL.md
• no automated test suite was run because this is a documentation-only change

[q107580018]

Closing this in favor of #901, which is already addressing Codex App worktree compatibility more comprehensively.