chore: enforce domain folder naming

AGENTS.md

@@ -251,7 +251,7 @@ OpenPencil follows a Reka UI-inspired component namespace structure:
 - Non-component domain folders use lowercase or kebab-case: `scene-graph/`, `figma-api/`, `node-edit/`.
 - Non-component TypeScript files use lowercase or kebab-case unless they are conventional entrypoints such as `index.ts`, `types.ts`, `context.ts`, or `use.ts`.
 - Multi-file root components live inside their component namespace folder, not beside it.
-- Use subfolders for multi-file domains instead of sibling files with repeated prefixes. Prefer `selection/container.ts`, `selection/hit-test.ts` over `selection-container.ts`, `selection-hit-test.ts`. When adding a second file for a domain (e.g. `eval-wrap.ts` next to `eval.ts`), create the folder immediately (`eval/index.ts` + `eval/wrap.ts`) instead of prefixing. The lint rule `no-sibling-domain-prefixed-files` catches this when a sibling folder exists, but the convention applies even before the folder is created.
+- Use subfolders for multi-file domains instead of sibling files with repeated prefixes. Prefer `selection/container.ts`, `selection/hit-test.ts` over `selection-container.ts`, `selection-hit-test.ts`. When adding a second file for a domain (e.g. `eval-wrap.ts` next to `eval.ts`), create the folder immediately (`eval/index.ts` + `eval/wrap.ts`) instead of prefixing. Oxlint catches sibling prefix files when a sibling folder exists; Steiger catches 3+ sibling files with the same prefix. The convention applies even before either rule triggers.
 
 - `@/` import alias for app cross-directory imports; app feature code lives under `src/app/*`
 - Use package-local aliases inside workspace packages: `#vue/*` in `packages/vue`, `#cli/*` in `packages/cli`, `#mcp/*` in `packages/mcp`, and `#core/*` when core code needs an alias. Prefer relative imports within nearby core modules when that is clearer than an alias.

scripts/steiger-rules.ts

@@ -17,6 +17,11 @@ type Rule = { name: string; check: (root: TreeEntry) => RuleResult }
 
 type FileRuleCheck = (sourceRel: string) => string | null
 
+const FILE_PREFIX_GROUP_ALLOWLIST = new Set([
+  'packages/core/src/lint/rules::no',
+  'tests/engine::visual'
+])
+
 type ImportRef = {
   specifier: string
   line: number
@@ -62,6 +67,13 @@ function collectFiles(entry: TreeEntry, files: string[] = []) {
   return files
 }
 
+function collectFolders(entry: TreeEntry, folders: TreeEntry[] = []) {
+  if (entry.type !== 'folder') return folders
+  folders.push(entry)
+  for (const child of entry.children ?? []) collectFolders(child, folders)
+  return folders
+}
+
 function importsIn(content: string): ImportRef[] {
   const imports: ImportRef[] = []
   const patterns = [
@@ -139,6 +151,40 @@ function createImportRule(
   }
 }
 
+function filePrefix(filePath: string): string | null {
+  const name = path.basename(filePath).replace(/\.(test|spec|bench)?\.?[cm]?[tj]sx?$|\.vue$/, '')
+  const match = /^([a-z][a-z0-9]+)-[a-z0-9-]+$/.exec(name)
+  return match?.[1] ?? null
+}
+
+const preferDomainFoldersOverFilenamePrefixes: Rule = {
+  name: 'open-pencil/prefer-domain-folders-over-filename-prefixes',
+  check(root) {
+    const diagnostics: Diagnostic[] = []
+    for (const folder of collectFolders(root)) {
+      const folderRel = relativePath(root.path, folder.path)
+      const groups = new Map<string, string[]>()
+      for (const child of folder.children ?? []) {
+        if (child.type !== 'file' || !TEXT_EXTENSIONS.has(path.extname(child.path))) continue
+        const prefix = filePrefix(child.path)
+        if (!prefix) continue
+        const files = groups.get(prefix) ?? []
+        files.push(child.path)
+        groups.set(prefix, files)
+      }
+      for (const [prefix, files] of groups) {
+        if (files.length < 3) continue
+        if (FILE_PREFIX_GROUP_ALLOWLIST.has(`${folderRel}::${prefix}`)) continue
+        diagnostics.push({
+          message: `Use a ${prefix}/ domain folder instead of ${files.length} sibling files with the ${prefix}- filename prefix.`,
+          location: { path: folder.path }
+        })
+      }
+    }
+    return { diagnostics }
+  }
+}
+
 const strictTestFilePlacement = createFileRule('open-pencil/strict-test-file-placement', (sourceRel) => {
   if (!sourceRel.startsWith('tests/')) return null
   if (!TEXT_EXTENSIONS.has(path.extname(sourceRel))) return null
@@ -350,6 +396,7 @@ const noUiImportsInCore = createImportRule(
 export const openPencilArchitecturePlugin = {
   meta: { name: 'open-pencil-architecture', version: '0.0.0' },
   ruleDefinitions: [
+    preferDomainFoldersOverFilenamePrefixes,
     strictTestFilePlacement,
     noEngineOnlyAssertionsInE2E,
     noE2EImportsInEngineTests,

steiger.config.ts

@@ -21,6 +21,7 @@ export default defineConfig([
   },
   {
     rules: {
+      'open-pencil/prefer-domain-folders-over-filename-prefixes': 'error',
       'open-pencil/strict-test-file-placement': 'error',
       'open-pencil/no-engine-only-assertions-in-e2e': 'error',
       'open-pencil/no-e2e-imports-in-engine-tests': 'error',