Add a native fish Module and set it as neogaia's login shell

Translate the CachyOS fish snapshot into a home-manager Module: the
fastfetch greeting, the bat-backed manpager, the done and bang-bang
plugins (from nixpkgs, not a plugin manager), the history/backup/copy
helper functions, and the eza and navigation aliases. Pacman-specific
aliases are dropped or replaced with NixOS equivalents (update ->
nixos-rebuild switch, cleanup -> nix-collect-garbage). Enabling the
Module also makes fish the user's default login shell.

.claude/tasks/0005-fish-shell-module.md

@@ -0,0 +1,34 @@
+---
+spec: laptop-mvi
+blocked-by: 0001-skeleton-and-building-host
+---
+
+## What to build
+
+A fish `Module`, configured natively via home-manager, that makes the shell feel like the current CachyOS setup but is correct for NixOS. Translate the existing `cachyos-config.fish` and the rest of the fish snapshot under `reference/home/.config/fish/`: the greeting, the bat-manpager, the `done` and bang-bang plugins, the helper functions, and the eza/nav aliases. Drop or replace every Arch/pacman-specific part with its NixOS equivalent. Set fish as the default login shell.
+
+Configure the plugins natively through home-manager rather than a fish plugin manager.
+
+## Acceptance criteria
+
+- [x] A fish `Module` (following the `Enable convention`) is enabled on `neogaia` and configured natively via home-manager.
+- [x] The greeting, bat-manpager, `done` and bang-bang plugins, helper functions, and eza/nav aliases from the reference config are reproduced.
+- [x] All Arch/pacman-specific parts are dropped or replaced with NixOS equivalents.
+- [x] fish is the user's default login shell.
+- [x] The `neogaia` toplevel still builds with the fish `Module` enabled.
+
+## Implementation Notes
+
+- **Plugins are native, not inlined.** `done` and `bang-bang` come from `pkgs.fishPlugins.*` via `programs.fish.plugins`; home-manager drops them into `~/.config/fish/conf.d/` where fish auto-sources them.
+  The reference config inlined the bang-bang `__history_previous_command` functions and binds by hand; the plugin supplies those, so they are not re-inlined.
+- **`done` tuning uses `set -g`, not `set -U`.** The reference set `__done_min_cmd_duration`/`__done_notification_urgency_level` as universal variables, which persist to the universal-variable file and then ignore config changes.
+  A declarative config must own these each session, so they are set global (`set -g`) in `interactiveShellInit`.
+- **Arch/pacman aliases dropped:** `grubup`, `fixpacman`, `mirror` (`cachyos-rate-mirrors`), `apt`/`apt-get` (`man pacman`), `big` (`expac`), `gitpkg`, `rip` (`expac`).
+  **Replaced with NixOS equivalents:** `update` → `sudo nixos-rebuild switch` (was `pacman -Syu`), `cleanup` → `sudo nix-collect-garbage -d` (was `pacman -Rns`).
+- **Machine-specific PATH hacks dropped, per spec story 6 ("no impure environment lookup").** The hardcoded `BUN_INSTALL`, the `node-v24...` tarball path, and `~/Applications/depot_tools` from `config.fish` are absolute/impure and are not reproduced; such tooling should be Nix-provided when its own Module arrives.
+  The portable bits are kept: `~/.local/bin` on PATH (guarded) and sourcing `~/.fish_profile`.
+- **`env.fish` and `rustup.fish` deliberately not carried over.** `ANDROID_HOME`/platform-tools (Android SDK) and `source ~/.cargo/env.fish` (rust) are dev-toolchain integrations outside the MVI's core tooling (fish/tmux/nvim/Claude Code); they belong to future per-toolchain Modules that provide those tools through Nix rather than sourcing an impure env file.
+- **`hw` (`hwinfo --short`) and `tb` (`nc termbin.com 9999`) dropped.** These are generic rather than pacman-specific, but each needs an extra package (`hwinfo`, a `netcat`) that the minimal install does not otherwise pull in; left out of the MVI and easy to add later.
+- **`copy` kept verbatim** (including the upstream `trim-right` call) to preserve exact parity with the current shell.
+- **Verification.** The `neogaia` system toplevel builds (the spec's primary seam).
+  The rendered `~/.config/fish/` was inspected in the build output: aliases, the three helper functions, the `fastfetch` greeting, the bat manpager, the `done` tuning vars, and the `conf.d/plugin-done.fish` + `conf.d/plugin-bang-bang.fish` plugin files are all present; `users.users.alexion.shell` resolves to `pkgs.fish`.