fix(zsh): defend the sourced zshrc against the /etc/zshrc defaults

/etc/zshrc renders promptInit and environment.shellAliases after
interactiveShellInit, so both overrode the shared zshrc: the prompt
became `prompt suse` and ls/ll/l fell back to coreutils. The plugin
sourcing in the zshrc points at the Arch path /usr/share/zsh/plugins
and silently found nothing.

Clear promptInit, drop the three aliases with null, and enable the
syntax-highlighting and autosuggestions modules.

DECISIONS.md

@@ -1,5 +1,52 @@
 # Decisions
 
+## 2026-08-20 – The sourced zshrc is defended against the /etc/zshrc defaults
+
+- **Who**: Dominik, ClaudeCode
+- **Why**: `programs.zsh.interactiveShellInit` sources the shared
+  `defaults/shell/zshrc`, but the shell on the test VM did not look like the one
+  on Arch. The file is not at fault — the pinned `moonarch` input is the same
+  revision as the working tree and the file is byte-identical. The generated
+  `/etc/zshrc` is: `nixos/modules/programs/zsh/zsh.nix` places
+  `interactiveShellInit` at line 277, then `environment.shellAliases` at 285,
+  then `promptInit` at 288. Three defaults therefore land after our file and
+  win. (1) `promptInit` defaults to `autoload -U promptinit && promptinit &&
+  prompt suse`, and `prompt suse` assigns `PS1` — the same parameter as
+  `PROMPT` — so the Catppuccin prompt was replaced by the SUSE one. (2)
+  `environment.shellAliases` carries `ls`, `ll` and `l` as `mkDefault`
+  (`nixos/modules/config/shells-environment.nix:235`), shadowing the eza
+  aliases. (3) Unrelated to ordering: the zshrc sources zsh-syntax-highlighting
+  and zsh-autosuggestions from `/usr/share/zsh/plugins`, the path of the Arch
+  packages. The lookups are `[[ -f ]]`-guarded, so both plugins were silently
+  absent.
+- **Tradeoffs**: (1) **Where the prompt is owned** — moving the prompt into
+  `promptInit` would place it in the slot NixOS intends for it, at the price of
+  splitting one shared file across two options and diverging from Arch. Setting
+  `promptInit = ""` keeps the zshrc the single owner. (2) **Alias removal** —
+  redefining `ls`/`ll`/`l` in Nix would duplicate the eza commands in a second
+  place, where they can drift from the zshrc. `null` removes the alias
+  (`zsh.nix:20` filters null values out) and leaves the definition in one file.
+  Scoped to `programs.zsh.shellAliases` rather than `environment.shellAliases`,
+  so bash keeps the coreutils `ls` — it has no eza aliases to fall back on. (3)
+  **Plugins** — a `/usr/share/zsh/plugins` symlink farm via tmpfiles would make
+  the existing lookups succeed and need no Nix-side option, but it rebuilds an
+  FHS path for a file that the NixOS modules already know how to source. The
+  modules were chosen, as with the polkit exception the other way around
+  (2026-08-07).
+- **How**: `modules/desktop.nix` — `programs.zsh.promptInit = ""`,
+  `programs.zsh.shellAliases` with `ls`/`ll`/`l` set to `null`,
+  `programs.zsh.syntaxHighlighting.enable` and
+  `programs.zsh.autosuggestions.enable` with `strategy = [ "history"
+  "completion" ]`, the value the zshrc sets on Arch. Verified by evaluating
+  `environment.etc.zshrc.text` for the `testvm` host before and after: the
+  prompt and alias blocks are now empty, and both plugins are sourced from the
+  store.
+- **Remaining exception**: the zshrc's `orphans` and `uninstall` aliases call
+  `pacman`. They are dead on NixOS but harmless, and rewriting them per distro
+  would fork the shared file.
+- **Not changed**: `compinit` runs twice — once from `enableGlobalCompInit`
+  (`zsh.nix:266`), once in the zshrc. Duplicated work, no defect, left alone.
+
 ## 2026-08-20 – Installing from an own ISO, one command
 
 - **Who**: Maintainer, ClaudeCode

modules/desktop.nix

@@ -129,6 +129,32 @@ in
     source ${moonarchSrc}/defaults/shell/zshrc
   '';
 
+  # /etc/zshrc places promptInit after interactiveShellInit, and its default
+  # ends in `prompt suse`, which assigns PS1 — the same parameter as PROMPT.
+  # Left at the default it overwrites the prompt the sourced zshrc just set.
+  # The zshrc owns the prompt, so nothing else has to run here.
+  programs.zsh.promptInit = "";
+
+  # The zshrc sources both plugins from /usr/share/zsh/plugins, the path the
+  # Arch packages use; the lookups are guarded and silently find nothing here.
+  # These modules source the same plugins from the store instead. The strategy
+  # is the one the zshrc sets on Arch.
+  programs.zsh.syntaxHighlighting.enable = true;
+  programs.zsh.autosuggestions = {
+    enable = true;
+    strategy = [ "history" "completion" ];
+  };
+
+  # environment.shellAliases carries ls, ll and l as defaults, and /etc/zshrc
+  # renders them after interactiveShellInit, so they shadow the eza aliases of
+  # the sourced zshrc. null drops an alias. Scoped to zsh: bash keeps the
+  # coreutils ls, which is the only thing it has.
+  programs.zsh.shellAliases = {
+    ls = null;
+    ll = null;
+    l = null;
+  };
+
   # --- File manager ---
   #
   # Counterpart to thunar, thunar-volman, thunar-archive-plugin, tumbler and