fix: remove towerops_nif from auto-compilers list

The Mix.Tasks.Compile.ToweropsNif module is part of the project source
and isn't on the code path when Mix first resolves compilers in a fresh
build — causes 'could not be found' in CI. NIF compilation is handled by:
- nix/shell.nix shellHook for dev (make -C c_src)
- nix/build.nix preBuild for CI/prod (injects towerops-nif package .so)
Run 'mix compile.towerops_nif' manually when changing C source in dev.

mix.exs

@@ -10,7 +10,13 @@ defmodule Towerops.MixProject do
       start_permanent: Mix.env() == :prod,
       aliases: aliases(),
       deps: deps(),
-      compilers: [:towerops_nif, :phoenix_live_view] ++ Mix.compilers(),
+      # NIF compiled by `make -C c_src` in dev shell (nix/shell.nix) and
+      # pre-built by `towerops-nif` Nix package in CI/prod. Not an auto-compiler
+      # because Mix.Tasks.Compile.ToweropsNif is part of the project source and
+      # isn't available on the code path when Mix first resolves the compilers
+      # list in a fresh build. Run `mix compile.towerops_nif` manually when
+      # changing C source between `mix compile` runs in dev.
+      compilers: [:phoenix_live_view] ++ Mix.compilers(),
       prune_code_paths: false,
       listeners: [Phoenix.CodeReloader],
       dialyzer: dialyzer(),