flake: format the tree with one treefmt

The pre-commit hooks kept their own list of formatters, so nixpkgs-fmt in
a hook and nixfmt in the check disagreed the moment flake-checks moved.
`nix fmt` is now built from the same treefmt module the formatting check
is, and the hook and the Makefile both call it.

.pre-commit-config.yaml

@@ -32,21 +32,15 @@ repos:
   # Local hooks for project-specific tooling
   - repo: local
     hooks:
-      # nixfmt for Nix files; the same formatter the flake's treefmt check
-      # runs, so the hook and the check cannot disagree.
-      - id: nixfmt
-        name: nixfmt
-        entry: nixfmt
-        language: system
-        files: \.nix$
-
-      # Prettier for formatting
-      - id: prettier
-        name: prettier
-        entry: prettier --write --list-different
+      # One formatter for Nix and markup, built from the same treefmt module as
+      # the flake's formatting check, so the hook and the check cannot disagree.
+      # Go is absent on purpose: golangci-lint owns it, below.
+      - id: treefmt
+        name: treefmt
+        entry: nix fmt --
         language: system
         exclude: ^docs/
-        types_or: [javascript, jsx, ts, tsx, yaml, json, html, css, scss, sass, markdown]
+        types_or: [javascript, jsx, ts, tsx, yaml, json, html, css, scss, sass, markdown, nix]
 
       # mdformat for docs
       - id: mdformat

.prettierignore

@@ -1,2 +1,5 @@
 .github/workflows/test-integration-v2*
 docs/
+
+# Golden fixtures are byte-compared; a formatter must not rewrite them.
+hscontrol/testdata/

AGENTS.md

@@ -109,7 +109,8 @@ prek run --all-files    # run hooks on the full tree
 
 Hooks cover: file hygiene (trailing whitespace, line endings, BOM),
 syntax validation (JSON/YAML/TOML/XML), merge-conflict markers, private
-key detection, nixpkgs-fmt, prettier, and `golangci-lint` via
+key detection, `treefmt` (the same formatter `nix fmt` and the flake's
+formatting check run), `mdformat` for `docs/`, and `golangci-lint` via
 `--new-from-rev=HEAD~1` (see the golangci-lint hook in .pre-commit-config.yaml). A manual
 invocation with an `upstream/main` remote is equivalent:
 

Makefile

@@ -50,7 +50,7 @@ test: check-deps $(GO_SOURCES) go.mod go.sum
 
 # Formatting targets
 .PHONY: fmt
-fmt: fmt-go fmt-mdformat fmt-prettier
+fmt: fmt-go fmt-mdformat fmt-tree
 
 .PHONY: fmt-go
 fmt-go: check-deps $(GO_SOURCES)
@@ -63,10 +63,10 @@ fmt-mdformat: check-deps
 	@echo "Formatting documentation..."
 	mdformat docs/
 
-.PHONY: fmt-prettier
-fmt-prettier: check-deps $(PRETTIER_SOURCES)
-	@echo "Formatting markup and config files..."
-	prettier --write '**/*.{ts,js,md,yaml,yml,sass,css,scss,html}'
+.PHONY: fmt-tree
+fmt-tree: check-deps $(PRETTIER_SOURCES)
+	@echo "Formatting Nix, markup and config files..."
+	nix fmt
 
 # Linting targets
 .PHONY: lint

flake.nix

@@ -278,6 +278,17 @@
           drv = pkgs.headscale;
         };
 
+        # `nix fmt` is built from the same treefmt module as the formatting
+        # check, so the two cannot disagree. Every formatter the tree is judged
+        # by lives here, not in a hand-kept list of pre-commit hooks.
+        formatter = fc.formatter (
+          common
+          // {
+            goFmt = "off";
+            prettier = true;
+          }
+        );
+
         checks = {
           headscale = pkgs.testers.nixosTest (import ./nix/tests/headscale.nix);
         }