fix(design): reload guard rejects directory paths

design/src/serve.ts:200-212 used to accept a path that resolved to the
allowedDir itself (the OR branch `|| resolvedReload === allowedDir`),
which then crashed readFileSync with EISDIR. Now:

  1. startsWith(allowedDir + path.sep) must pass — rejects the dir itself
     and anything outside (403).
  2. statSync(resolvedReload).isFile() must pass — rejects subdirectories
     inside allowedDir with a clear "Path must be a file" 400.

The test stub in serve.test.ts mirrors prod; both updated, plus two new
test cases for the previously-broken paths. Codex caught this in the
plan-review pass; it's a latent bug in shipping code, not a regression
from the daemon work.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

design/src/serve.ts

@@ -197,19 +197,25 @@ export async function serve(options: ServeOptions): Promise<void> {
       );
     }
 
-    // Security: resolve symlinks and validate the reload path is within the
-    // allowed directory (anchored to the initial HTML file's parent).
-    // Prevents path traversal via /api/reload reading arbitrary files.
+    // Security: resolve symlinks and validate the reload path is a FILE
+    // inside the allowed directory (anchored to the initial HTML file's
+    // parent). Prevents path traversal via /api/reload reading arbitrary
+    // files. A path resolving to the allowedDir itself (a directory) used
+    // to pass the guard and then crash readFileSync with EISDIR — reject
+    // it explicitly with a clear 400 instead.
     const resolvedReload = fs.realpathSync(path.resolve(newHtmlPath));
-    if (
-      !resolvedReload.startsWith(allowedDir + path.sep) &&
-      resolvedReload !== allowedDir
-    ) {
+    if (!resolvedReload.startsWith(allowedDir + path.sep)) {
       return Response.json(
         { error: `Path must be within: ${allowedDir}` },
         { status: 403 },
       );
     }
+    if (!fs.statSync(resolvedReload).isFile()) {
+      return Response.json(
+        { error: `Path must be a file, not a directory: ${newHtmlPath}` },
+        { status: 400 },
+      );
+    }
 
     // Swap the HTML content
     htmlContent = fs.readFileSync(resolvedReload, "utf-8");