Restore dotfiles upgrade check while keeping git auto-fetch disabled

[MatthewDolan]

Motivation

• Re-enable the periodic dotfiles upgrade check so users continue to receive daily upgrade prompts.
• Keep automatic git fetch behavior disabled to avoid unexpected network activity from the shell.

Description

• Reinserted the _dotfiles_check_for_upgrade function and its invocation into home/.zshrc to restore the once-per-24-hour upgrade check.
• Left the git-auto-fetch plugin out of the Oh My Zsh plugins list to prevent automatic fetching across repositories.
• Only home/.zshrc was modified.

Testing

• Sourced the Hermit environment and ran the project test suite with . ./bin/activate-hermit && ./ci/test.sh, and all automated tests passed (1..13, all ok).

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Codex Task