Use robust shell escaping based on Python's shlex.quote()

[ianks]

Simplify shell_escape() to always wrap strings in single quotes, matching Python's shlex.quote() approach. In POSIX shells, single quotes protect all characters except the single quote itself, so newlines and other special characters no longer need rejection—they're literal inside single quotes.

[Copilot]

Pull request overview

This PR refactors the shell_escape() function to adopt Python's shlex.quote() approach, where strings are wrapped in single quotes with proper escaping of embedded single quotes.

Key Changes:

• Moved shell_escape() implementation earlier in the file (before its first use) and updated it to wrap strings in single quotes, making all characters literal except single quotes themselves
• Updated all shell script builders (build_cd_script, build_mkdir_script, build_clone_script, build_delete_script, build_worktree_script) to use the new escaping function and removed manual quotes from format strings
• Applied shell escaping to paths in cmd_init() for both Fish and Bash/Zsh shell function generation

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