feat(installer): enrich brew deps for fish on Arch

[AdrianLinares]

Depends on #178 (stacked). Once #178 merges, GitHub will auto-resolve this PR to show only the Fish-block Brew enrichment + Arch-empty change. Until then the diff appears to include the carapace removal from Fish/Zsh/Nushell Arch — that is intentional (this branch is built on top of #178) and will disappear automatically.

Problem

Two coupled issues for the Fish shell installation path:

1. The README documents brew install fish fisher starship zoxide atuin fzf carapace nvm rustup-init coreutils as the full set of fish dependencies on macOS/Linux (README.md §2.3). The installer's Brew field only listed fish carapace zoxide atuin starship, so fisher, fzf, coreutils, nvm, and rustup-init were silently skipped — users had to install them manually post-install to get the documented fish experience.
2. On Arch, the fish-shell path is meant to go through Homebrew (README.md §2.3 — .tmux.conf hard-codes /home/linuxbrew/.linuxbrew/bin/fish as the default shell), but the installer still tried pacman -S fish …, producing a partial install that doesn't match what the README describes.

Root cause

installer/internal/tui/installer.go (Fish block):

Termux: "fish starship zoxide",
Brew:   "fish carapace zoxide atuin starship",   // ← README lists more
Arch:   "fish zoxide atuin starship",             // ← should let brew fall through
Fedora: "fish carapace zoxide atuin starship",

The Brew field is missing five packages from the README's documented list (fisher, fzf, coreutils, nvm, rustup-init). The Arch field non-empty value makes runNativeWithBrewFallback run pacman first and treat the native install as success, never reaching brew.

Fix

Two field changes, Fish shell block only:

Field	Before	After
Brew	fish carapace zoxide atuin starship	fish fisher starship zoxide atuin fzf carapace nvm rustup-init coreutils (matches README)
Arch	fish zoxide atuin starship	"" (empty → brew fallback)

Zsh and Nushell blocks are untouched. Their Brew fields (which already include carapace) and their Arch fields (which already exclude carapace after #178) remain unchanged.

Verification

$ cd installer && go build ./...      # ok
$ cd installer && go vet ./internal/tui/   # ok
$ cd installer && go test ./...
ok      github.com/Gentleman-Programming/Gentleman.Dots/installer/internal/tui/trainer
FAIL    github.com/Gentleman-Programming/Gentleman.Dots/installer/internal/tui

The single failing test (internal/tui) is an environmental golden-file mismatch — the golden expects Detected: macOS but this machine runs Arch Linux. It fails identically on pristine main and on the base of #178 (without either patch). Unrelated to this PR.

Stacking note

• Built directly on top of fix(installer): carapace not in pacman repos on Arch #178 (fix/arch-carapace-package-not-found).
• If a maintainer prefers this un-stacked, I can rebase onto main — but then the carapace removal from the Fish Arch field would reappear in this diff until fix(installer): carapace not in pacman repos on Arch #178 lands. Stacking is the cleaner story here.