docs: correct the home.file key gotcha — keyed by the writer's path form, not always absolute

Firefox writes ~/.config/mozilla/firefox/profiles.ini itself on first launch,
pointing at an auto-created random-prefix profile. Home-manager then refuses to
clobber that file and fails the whole activation, so the declarative default
profile — search.json.mozlz4 (DuckDuckGo) and the Stylix user.js — never lands
and Firefox keeps running the stale profile with Google as default and no theme.

Set force = true on the generated profiles.ini home.file entry so home-manager
owns it, deploys the default profile, and Firefox switches to it.

CLAUDE.md

@@ -57,8 +57,9 @@ The domain model (Host, Module, Skeleton, Auto-loader, Enable convention, overla
   Editing a skill in place fails; its source is `modules/agents/claude-code/skills/<name>/` here, applied by a rebuild.
   Creating a new file under `~/.claude/skills/` succeeds silently and is the trap — it stays outside the repo and reaches no other machine.
   Copying out of that tree needs `cp -rL` plus `chmod -R u+w`: a plain `cp -r` copies the symlinks, putting store paths into the destination, and dereferenced files keep the store's read-only mode.
-- `home-manager.users.<user>.home.file` is keyed by **absolute** path, not by a path relative to the home directory.
-  Evaluating `home.file.".claude/CLAUDE.md"` fails with "does not provide attribute"; the working key is `home.file."/home/alexion/.claude/CLAUDE.md"`.
+- `home-manager.users.<user>.home.file` is keyed by whatever path string the **defining module wrote**, absolute or relative, not by one canonical form.
+  A module that writes `home.file."/home/alexion/.claude/CLAUDE.md"` is reachable only at that absolute key, while `programs.firefox` writes relative keys such as `home.file.".config/mozilla/firefox/profiles.ini"` reachable only at the relative form.
+  The other form fails with "does not provide attribute", so overriding an entry (e.g. setting `.force = true` on it) requires matching the writer's exact key.
   List the real keys with `nix eval --json .#nixosConfigurations.<host>.config.home-manager.users.<user>.home.file --apply builtins.attrNames` rather than guessing one.
   A key's `.source` is the input file, whose store path differs from the deployed symlink's target (home-manager copies it to a `hm_`-prefixed path) even though the contents match.
 - nixpkgs `vimPlugins.nord-nvim` is `shaunsingh/nord.nvim` (no `require("nord").setup()`); the config wants `gbprod/nord.nvim`, which is packaged as `vimPlugins.gbprod-nord`.

modules/desktop/firefox.nix

@@ -15,7 +15,7 @@ in
   options.modules.desktop.firefox.enable = lib.mkEnableOption "Firefox as the desktop browser";
 
   config = lib.mkIf cfg.enable {
-    home-manager.users.${user} = {
+    home-manager.users.${user} = hm: {
       programs.firefox = {
         enable = true;
 
@@ -90,6 +90,10 @@ in
           "x-scheme-handler/https" = "firefox.desktop";
         };
       };
+
+      # Firefox writes profiles.ini itself on first launch, so home-manager is
+      # told to own the file rather than fail activation refusing to clobber it.
+      home.file."${hm.config.programs.firefox.configPath}/profiles.ini".force = true;
     };
   };
 }