feat(installer): support atomic/immutable Linux distros

[blak0p]

Closes #165

Summary

• Auto-detect atomic/immutable Linux distros (Fedora Silverblue, Bazzite, Kinoite, etc.)
• Prefer Homebrew over sudo dnf/apt on atomic systems (brew works in userspace)
• Use flatpak for WezTerm installation
• Skip sudo-based shell changes on atomic (/etc/shells is read-only)
• Show manual install instructions when brew/flatpak aren't available

Changes

File	Change
installer/internal/system/detect.go	Add IsAtomic, HasFlatpak fields; checkAtomic(), checkFlatpak() functions
installer/internal/system/exec.go	Add RunRpmOstree, RunFlatpak helpers
installer/internal/system/detect_test.go	Tests for atomic detection
installer/internal/tui/installer.go	Atomic-aware step functions (brew-first, skip sudo)
installer/internal/tui/interactive.go	Atomic-aware interactive scripts
installer/internal/tui/model.go	Atomic-aware SetupInstallSteps, ManualPackages field
installer/cmd/gentleman-installer/main.go	Show atomic mode in non-interactive output

Test Plan

• go build ./... compiles without errors
• go test ./internal/system/ -count=1 -short passes
• Manually tested on Bazzite (Fedora Atomic) with fish + tmux + ghostty + nvim + font
• Config files copy correctly to $HOME
• Homebrew installs packages in userspace
• Flatpak installs WezTerm when available
• Shell change step shows manual instructions instead of sudo

[masch]

Hi @Alan-TheGentleman! Do you plan to merge this feature into the main branch? It would be great if the project could support immutable Linux distros.

[masch]

Hi @blak0p! I use Fedora Silverblue. Do your changes work on it? I'd be happy to test them out if they do.

[blak0p]

Hey @masch! Yes, it works. It's built around rpm-ostree in general, not anything Bazzite-specific, so Silverblue should be fine. If you test it and run into anything weird, let me know here.