fix: cache tmp dir resolution, add test for TTS temp path

Address review feedback:
- Cache resolvePreferredOpenClawTmpDir() result to avoid sync I/O
  on every media path check and prevent mkdir side effects
- Add test for TTS voice output from /tmp/openclaw/ path

src/auto-reply/reply/reply-media-paths.test.ts

@@ -176,4 +176,24 @@ describe("createReplyMediaPathNormalizer", () => {
       mediaUrls: ["/Users/peter/.openclaw/media/outbound/persisted.png"],
     });
   });
+
+  it("keeps TTS voice output from the OpenClaw temp directory", async () => {
+    // resolvePreferredOpenClawTmpDir() returns /tmp/openclaw on POSIX when it exists.
+    // We rely on the real function (no mock) since the test environment has /tmp/openclaw.
+    const normalize = createReplyMediaPathNormalizer({
+      cfg: {},
+      sessionKey: "session-key",
+      workspaceDir: "/tmp/agent-workspace",
+    });
+
+    const result = await normalize({
+      mediaUrls: ["/tmp/openclaw/tts-abc123/voice-1234567890.opus"],
+    });
+
+    expect(result).toMatchObject({
+      mediaUrl: "/tmp/openclaw/tts-abc123/voice-1234567890.opus",
+      mediaUrls: ["/tmp/openclaw/tts-abc123/voice-1234567890.opus"],
+    });
+    expect(saveMediaSource).not.toHaveBeenCalled();
+  });
 });

src/auto-reply/reply/reply-media-paths.ts

@@ -60,10 +60,12 @@ function isAllowedAbsoluteReplyMediaPath(params: {
  * These are trusted paths written by OpenClaw's own tooling
  * (TTS, media processing, etc.) and should be deliverable as reply media.
  */
+let cachedTmpRoot: string | undefined;
+
 function isOpenClawTmpPath(candidate: string): boolean {
   try {
-    const tmpRoot = resolvePreferredOpenClawTmpDir();
-    return isPathInside(tmpRoot, candidate);
+    cachedTmpRoot ??= resolvePreferredOpenClawTmpDir();
+    return isPathInside(cachedTmpRoot, candidate);
   } catch {
     return false;
   }