Address PR #40 review feedback

- examples/10-toilet-slacking-base.png and examples/11-touch-fish-base.png were
  JPEGs under a .png extension; the docs ask the agent to pass them as
  image_synthesize input_file_paths, so the host may sniff by extension and
  refuse. Re-encode both to real PNGs (same paths, no doc churn).

- make_preview_strip.py: when --workdir points at an existing regular file,
  os.makedirs(..., exist_ok=True) used to leak an uncaught FileExistsError
  traceback. Reject that case in argument validation with a clear message.

- smoke.test.mjs: add two regressions — PNG signature check on the three
  examples/, and a subprocess invocation that drives the workdir regression
  through spawnSync.

Author: weekbin

plugins/weekbin/octopus-meme-maker/examples/10-toilet-slacking-base.png

@@ -103,10 +103,16 @@ def main():
             sys.exit(1)
 
     # A caller-supplied --workdir is never deleted: refusing a non-empty one is
-    # cheaper than explaining why the user's files are gone.
+    # cheaper than explaining why the user's files are gone. We also refuse a path
+    # that already exists as a regular file — `os.makedirs(..., exist_ok=True)`
+    # would raise an uncaught `FileExistsError` and leak a Python traceback.
     cleanup = None
     if args.workdir:
         workdir = os.path.abspath(args.workdir)
+        if os.path.exists(workdir) and not os.path.isdir(workdir):
+            print(f"ERROR: --workdir {workdir} is not a directory. Pass a directory path, or omit "
+                  "--workdir to use a temporary one.", file=sys.stderr)
+            sys.exit(1)
         if os.path.isdir(workdir) and os.listdir(workdir):
             print(f"ERROR: --workdir {workdir} is not empty. Pass an empty directory, or omit "
                   "--workdir to use a temporary one.", file=sys.stderr)

plugins/weekbin/octopus-meme-maker/examples/11-touch-fish-base.png

@@ -9,7 +9,7 @@
 
 import { test } from 'node:test';
 import assert from 'node:assert/strict';
-import { readFileSync, existsSync, statSync, readdirSync, writeFileSync, mkdtempSync, mkdirSync, rmSync, unlinkSync } from 'node:fs';
+import { readFileSync, existsSync, statSync, readdirSync, writeFileSync, mkdtempSync, mkdirSync, rmSync, unlinkSync, openSync, readSync, closeSync } from 'node:fs';
 import { join, resolve } from 'node:path';
 import { tmpdir } from 'node:os';
 import { spawnSync } from 'node:child_process';
@@ -307,6 +307,26 @@ test('examples/ has 3 base.png samples', () => {
   }
 });
 
+// The host may sniff the MIME by extension, so the file content must match the
+// .png suffix. (Reviewer feedback: these used to be JPEGs under a .png name and
+// the docs asked the agent to pass them to image_synthesize.)
+test('examples/*.png are actually PNG (signature 89 50 4E 47 0D 0A 1A 0A)', () => {
+  const PNG_SIG = Buffer.from([0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a]);
+  for (const f of ['02-stay-late-base.png', '10-toilet-slacking-base.png', '11-touch-fish-base.png']) {
+    const fd = openSync(join(EX_DIR, f), 'r');
+    try {
+      const head = Buffer.alloc(8);
+      readSync(fd, head, 0, 8, 0);
+      assert.ok(
+        head.equals(PNG_SIG),
+        `${f} is not a real PNG (got bytes ${[...head].map((b) => b.toString(16).padStart(2, '0')).join(' ')})`,
+      );
+    } finally {
+      closeSync(fd);
+    }
+  }
+});
+
 // ---------------------------------------------------------------------------
 // 6b. Host-tool contract vocabulary.
 //
@@ -545,6 +565,37 @@ test('make_gif.py refuses output names that cannot work on Windows', () => {
   rmSync(base, { recursive: true, force: true });
 });
 
+test('make_preview_strip.py rejects --workdir pointing at a regular file (no traceback)', () => {
+  // Reviewer feedback (PR #40): when --workdir resolves to an existing regular
+  // file, `os.makedirs(..., exist_ok=True)` used to leak an uncaught
+  // FileExistsError traceback. The script must fail with a clear argument
+  // error instead.
+  if (pythonHasModule('os') !== true) return;
+
+  const tmp = mkdtempSync(join(tmpdir(), 'octopus-workdir-'));
+  const fakeFile = join(tmp, 'not-a-dir.txt');
+  writeFileSync(fakeFile, 'placeholder');
+  const video = join(REF_DIR, 'videos', 'breakdown-h3.mp4');
+  const out = join(tmp, 'preview.png');
+
+  const r = spawnSync(
+    'python3',
+    [join(SCRIPTS_DIR, 'make_preview_strip.py'), video, out,
+     '--workdir', fakeFile, '--labels', 'a,b,c,d,e'],
+    { encoding: 'utf8' },
+  );
+
+  assert.notEqual(r.status, 0, 'script must exit non-zero on a non-directory --workdir');
+  assert.match(r.stderr, /--workdir/, 'stderr must name --workdir in the error');
+  assert.match(r.stderr, /not a directory/, 'stderr must explain the path is not a directory');
+  assert.doesNotMatch(
+    r.stderr,
+    /Traceback|FileExistsError/,
+    'stderr must not leak a Python traceback',
+  );
+  rmSync(tmp, { recursive: true, force: true });
+});
+
 test('path containment is compared with the platform normalisation', () => {
   // Windows is case-insensitive and accepts both separators, so a raw string
   // comparison of dirname(resolved) against the scene dir is wrong there.