chore: add a formatter, matching the style this repo already writes

singleQuote=true was chosen by counting this repo's own
imports, not by fleet decree. The fleet is genuinely split and the two repos
that already had a .prettierrc disagreed with each other, so there was no
standard to restore. Quote style does not cross repo boundaries; having a
gate does.

Markdown is ignored for now — prettier rewraps prose, which would bury the
real diff.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: catomean

.prettierignore

@@ -0,0 +1,31 @@
+# Build output and vendored trees — formatting these is noise.
+node_modules
+.next
+dist
+build
+out
+coverage
+.turbo
+.vercel
+*.min.js
+*.min.css
+
+# Generated during a build, so it is absent locally and present in CI — which
+# makes a clean local --check no evidence at all. Contentlayer's output also
+# uses import assertions, which prettier's parser rejects outright.
+.contentlayer
+.astro
+.svelte-kit
+storybook-static
+test-results
+playwright-report
+
+# Lockfiles are generated; prettier would rewrite them wholesale.
+package-lock.json
+pnpm-lock.yaml
+yarn.lock
+
+# Markdown is deliberately out of scope for now. Prettier rewraps prose, which
+# is where it is most opinionated and least useful, and it would bury the real
+# diff. Remove this line when you want docs formatted too.
+*.md

.prettierrc

@@ -0,0 +1,9 @@
+{
+  "semi": true,
+  "singleQuote": true,
+  "printWidth": 100,
+  "tabWidth": 2,
+  "trailingComma": "all",
+  "arrowParens": "always",
+  "endOfLine": "lf"
+}

package-lock.json

@@ -44,14 +44,17 @@
     "lint": "eslint .",
     "typecheck": "tsc -p tsconfig.json --noEmit",
     "test": "node --test test/*.test.js",
-    "verify": "npm run lint && npm run typecheck && npm run build && npm test",
-    "prepare": "npm run build"
+    "verify": "npm run format:check && npm run lint && npm run typecheck && npm run build && npm test",
+    "prepare": "npm run build",
+    "format": "prettier --write .",
+    "format:check": "prettier --check ."
   },
   "devDependencies": {
     "@eslint/js": "^9.39.5",
     "@types/node": "^22.10.2",
     "eslint": "^9.39.5",
     "globals": "^15.15.0",
+    "prettier": "3.9.6",
     "typescript": "^5.8.2",
     "typescript-eslint": "^8.67.0"
   }