fix(config-protection): protect shared/base linter configs, not just entry points

`PROTECTED_FILES` matches exact basenames, so it only ever guarded a tool's
canonical entry point. Real repos split flat config across files — a shared
`eslint.config.base.mjs` holding the ignore list and rule severities, imported
by per-workspace `eslint.config.mjs` files, is the common monorepo shape.

That meant the hook protected the leaves and left the trunk wide open:

    eslint.config.base.mjs        <- ignore list + rule severities   UNPROTECTED
    frontend/eslint.config.mjs    <- imports the base                protected
    backend/eslint.config.mjs     <- imports the base                protected

An agent blocked from touching the two leaves could silently rewrite every rule
severity in the file they both import. Hit in practice: two edits to
`eslint.config.js` were correctly blocked, then an edit to
`eslint.config.base.mjs` in the same repo went through unchallenged.

Adds `PROTECTED_PATTERNS` alongside the existing Set, covering
`<tool>.config.<qualifier>.<ext>` and `.<tool>rc.<qualifier>.<ext>` for the
linters and formatters already listed. Case-insensitive, for the same reason
the Set lookup is (#2543).

Deliberately NOT matched: `vite.config.ts`, `vitest.config.ts`,
`jest.config.js`, `playwright.config.ts`, `tsconfig.json`. This hook exists to
stop a LINTER config being weakened in place of fixing the code; editing a
bundler or test-runner config is ordinary work, and sweeping those in would
make the hook obstructive. A second test pins that boundary so a future
widening of the patterns cannot quietly cross it.

The exact-name Set is untouched, so nothing previously protected becomes
unprotected, and first-time creation stays allowed (the bootstrap path).

Tests: 11 pass. The new regression test was verified failing against the
unpatched hook first; the boundary test passes either way by design and is
there as the control.

scripts/hooks/config-protection.js

@@ -62,6 +62,40 @@ const PROTECTED_FILES = new Set([
   '.markdownlintrc'
 ]);
 
+/**
+ * Exact basenames only catch a tool's canonical entry point. Real repos split
+ * flat config across files: a shared `eslint.config.base.mjs` holding the
+ * ignore list and rule severities, imported by per-workspace
+ * `eslint.config.mjs` files. That is the common monorepo shape, and matching
+ * basenames alone protected the leaves while leaving the trunk — the file that
+ * actually carries the rules — freely editable.
+ *
+ * These patterns cover `<tool>.config.<qualifier>.<ext>` and
+ * `.<tool>rc.<qualifier>.<ext>` for the linters and formatters listed above.
+ * They are case-insensitive for the same reason the Set lookup above is.
+ *
+ * Deliberately NOT matched: build and test tooling — `vite.config.ts`,
+ * `vitest.config.ts`, `jest.config.js`, `playwright.config.ts`,
+ * `tsconfig.json`. This hook exists to stop a LINTER config being weakened in
+ * place of fixing the code; editing a bundler or test-runner config is
+ * ordinary work, and sweeping those in would make the hook obstructive.
+ */
+const PROTECTED_PATTERNS = [
+  // eslint.config.base.mjs, prettier.config.shared.cjs, stylelint.config.local.js …
+  /^(eslint|prettier|stylelint|commitlint|oxlint|biome)\.config(\.[A-Za-z0-9_-]+)*\.(js|mjs|cjs|ts|mts|cts)$/i,
+  // .eslintrc.base.json, .prettierrc.shared.yml …
+  /^\.(eslintrc|prettierrc|stylelintrc|markdownlintrc)(\.[A-Za-z0-9_-]+)*\.(js|cjs|mjs|json|jsonc|yml|yaml|toml)$/i,
+  // biome.base.json, biome.shared.jsonc
+  /^biome(\.[A-Za-z0-9_-]+)*\.jsonc?$/i,
+];
+
+function isProtectedName(basename) {
+  const lower = basename.toLowerCase();
+  return PROTECTED_FILES.has(basename)
+    || PROTECTED_FILES.has(lower)
+    || PROTECTED_PATTERNS.some((re) => re.test(basename));
+}
+
 function parseInput(inputOrRaw) {
   if (typeof inputOrRaw === 'string') {
     try {
@@ -101,7 +135,7 @@ function run(inputOrRaw, options = {}) {
   // silently overwrite the real config while the guard returned exit 0.
   // On genuinely case-sensitive filesystems this only costs a false positive
   // on a distinct file that differs from a protected name by case alone.
-  if (PROTECTED_FILES.has(basename) || PROTECTED_FILES.has(basename.toLowerCase())) {
+  if (isProtectedName(basename)) {
     // Allow first-time creation — there's no existing config to weaken.
     // The hook's purpose is blocking modifications; writing a brand-new
     // config file in a project that has none is a legitimate bootstrap

tests/hooks/config-protection.test.js

@@ -357,6 +357,66 @@ function runTests() {
     passed++;
   else failed++;
 
+  if (
+    test('blocks shared/base flat configs, not just the canonical entry point', () => {
+      // Monorepos split flat config: a shared `eslint.config.base.mjs` holding
+      // the ignore list and rule severities, imported by per-workspace
+      // `eslint.config.mjs` files. Matching basenames alone protected the
+      // leaves and left the trunk -- the file that carries the rules -- editable.
+      const tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), 'ecc-config-protect-base-'));
+      try {
+        const names = ['eslint.config.base.mjs', 'prettier.config.shared.cjs', '.eslintrc.base.json', 'ESLint.Config.Base.MJS'];
+        for (const name of names) {
+          const absPath = path.join(tmpDir, name);
+          fs.writeFileSync(absPath, '{}');
+
+          const result = runHook({ tool_name: 'Edit', tool_input: { file_path: absPath } });
+
+          assert.strictEqual(result.code, 2, 'Expected ' + name + ' to be blocked');
+          assert.ok(
+            result.stderr.includes('BLOCKED: Modifying ' + name + ' is not allowed.'),
+            'Expected block message for ' + name + ', got: ' + result.stderr
+          );
+        }
+      } finally {
+        try {
+          fs.rmSync(tmpDir, { recursive: true, force: true });
+        } catch {
+          // best-effort cleanup
+        }
+      }
+    })
+  )
+    passed++;
+  else failed++;
+
+  if (
+    test('does not block build or test tooling configs', () => {
+      // Pins the boundary: this hook guards LINTER configs. A future widening
+      // of the patterns must not quietly start blocking ordinary work.
+      const tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), 'ecc-config-protect-allow-'));
+      try {
+        const names = ['vite.config.ts', 'vitest.config.ts', 'jest.config.js', 'playwright.config.ts', 'tsconfig.json'];
+        for (const name of names) {
+          const absPath = path.join(tmpDir, name);
+          fs.writeFileSync(absPath, '{}');
+
+          const result = runHook({ tool_name: 'Edit', tool_input: { file_path: absPath } });
+
+          assert.strictEqual(result.code, 0, 'Expected ' + name + ' to be allowed, stderr: ' + result.stderr);
+        }
+      } finally {
+        try {
+          fs.rmSync(tmpDir, { recursive: true, force: true });
+        } catch {
+          // best-effort cleanup
+        }
+      }
+    })
+  )
+    passed++;
+  else failed++;
+
   console.log(`\nResults: Passed: ${passed}, Failed: ${failed}`);
   process.exit(failed > 0 ? 1 : 0);
 }