Harden bash and zsh prompt rendering

[jdloft]

Motivation

• Narrow terminal windows and history/search navigation revealed prompt redraw/cursor-position corruption caused by unmarked non-printing ANSI sequences and zsh % expansions in dynamic fields.
• The prompt host color used the literal prompt escape token instead of the actual hostname which made special-case coloring (e.g. toolbox) unreliable.

Description

• Added a Bash helper _dotfiles-bash-np to wrap ANSI color sequences with explicit readline non-printing delimiters (\001/\002) and applied it to the main prompt, exit-code, git, virtualenv, k8s and chroot segments in source/bash/20_prompt.bash.
• Reworked Bash prompt-building to concatenate safe non-printing markers via _dotfiles-bash-np and updated _dotfiles-exit_code, _dotfiles-git-prompt, _dotfiles-virtualenv-prompt, and _dotfiles-k8s-prompt to use the helper.
• Fixed host detection in both Bash and Zsh to use a real hostname variable (host_name from DOTFILES_HOST/HOSTNAME) instead of comparing the literal prompt escape token, and made the chroot prompt resilient to being read once.
• Hardened Zsh prompt in source/zsh/20_prompt.zsh by switching to %~ for the cwd and escaping % characters in dynamic fields (chroot, git branch, virtualenv, k8s context) to avoid percent-expansion and width miscalculation.

Testing

• bash -n source/bash/20_prompt.bash completed successfully with no syntax errors.
• zsh -n source/zsh/20_prompt.zsh could not be run in this environment because zsh is not installed, so Zsh syntax validation was not performed automatically.

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Codex Task