Add Fish shell configuration

[tombell]

Summary

• add modular Fish environment, aliases, integrations, keybindings, and repository helpers
• preserve the existing Git and Jujutsu-aware prompt
• add macOS, Linux, and work-specific Fish configuration overlays

Validation

• syntax checked all Fish files with Fish 4.8.1
• verified fish_indent formatting
• verified aliases, integrations, and custom keybindings load
• verified Git and Jujutsu prompt rendering

Notes

• leaves the existing zsh configuration intact for migration and rollback
• omits tendr completion because the installed CLI currently supports only Bash and zsh