docs: explain how consumers override public module defaults

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CLAUDE.md

@@ -15,6 +15,20 @@ one as a flake input. Deploy happens from the private repo, not here.
 - secrets (keys, tokens, passwords) → **neither** repo; runtime files or a secrets manager
   decrypting to a path. See the SSH section.
 
+## Overrides = options (the consumer contract)
+
+Private imports these modules **read-only** (flake input) — it can't patch a package list
+or a hard-coded value in place. So anything a consumer might reasonably want to change is
+exposed as an **option with a default**, never baked into config. That is what makes this
+repo a *baseline others extend* rather than a fork-and-patch.
+
+Pattern (see `modules/texlive.nix`): option-bearing config lives in its own sub-module
+under the `local.*` namespace, `imports`-ed by `home.nix`. A module that declares `options`
+must use the split `{ options = …; config = …; }` form, so isolating it per file keeps
+`home.nix` bare. Override is then one line, e.g.
+`local.texlive.package = pkgs.texlive.withPackages (ps: [ ps.scheme-full ]);`. A plain value
+with nothing to vary goes straight in the bare package list — no option, don't over-abstract.
+
 ## Maxim: keep $HOME clean (XDG)
 
 Standing principle for every addition: minimise dotfiles/dirs in `$HOME`. Route a tool's