feat: add multi-shell architecture with Zsh support

[ki11e6]

Summary

Adds a shell-adapter architecture that enables Omakub to support multiple shells. This PR introduces Zsh as an alternative to Bash while maintaining full backward compatibility.

Key Changes

• Shell-agnostic shared configs (defaults/shared/):

  • environment.sh - PATH, exports, OMAKUB_PATH
  • aliases.sh - All command aliases
  • functions.sh - Utility functions (POSIX-compatible)

• Zsh adapter (defaults/zsh/):

  • shell - Zsh options with bash compatibility (KSH_ARRAYS, etc.)
  • rc - Sources shared configs + zsh-specific
  • init - Tool initialization (mise, zoxide, fzf)
  • prompt - Minimal Nerd Font prompt
  • completions.zsh - Tab completion + arrow key history search

• Bash adapter refactor (defaults/bash/):

  • Now sources shared configs
  • Removed duplicated aliases/functions

• Installer updates:

  • first-run-choices.sh - Shell selection during install
  • a-shell.sh - Dual-shell installation support

• CLI integration:

  • Added "Shell" option to omakub menu
  • shell.sh - Shell selection menu
  • shell-switch.sh - Switch logic with backup

Architecture

defaults/
├── shared/           # Shell-agnostic (both shells source these)
│   ├── environment.sh
│   ├── aliases.sh
│   └── functions.sh
├── bash/             # Bash-specific adapter
│   ├── rc            # Sources shared + bash-specific
│   ├── shell         # Bash options
│   └── ...
└── zsh/              # Zsh-specific adapter
    ├── rc            # Sources shared + zsh-specific
    ├── shell         # Zsh options (KSH_ARRAYS for bash compat)
    └── ...

Testing

Tested ✓

• Bash shell config loading
• Zsh shell config loading
• Syntax validation of all scripts (bash -n, zsh -n)
• mise activation (with KSH_ARRAYS workaround)
• starship prompt rendering in both shells
• Shell switching via omakub CLI menu
• Arrow key history search in zsh
• Shared aliases and functions in both shells
• app2folder-remove() function (fixed for POSIX compatibility)
• Full logout/login cycle after shell switch

Not Tested

• Fresh Omakub installation (first-run-choices.sh shell selection)
• Clean system without existing shell configs

Notes

• Shell switch requires logout/login for new terminals (standard chsh behavior)
• User customizations are backed up but not auto-migrated (user manually copies to avoid breaking shell-specific commands)
• mise required a workaround: temporarily disable KSH_ARRAYS during activation due to $+functions[...] syntax incompatibility