fix: guard resolveUserPath against undefined input (#10176)

* fix: guard resolveUserPath against undefined input

When subagent spawner omits workspaceDir, resolveUserPath receives
undefined and crashes on .trim().  Add a falsy guard that falls back
to process.cwd(), matching the behavior callers already expect.

Closes #10089

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: harden runner workspace fallback (#10176) (thanks @Yida-Dev)

* fix: harden workspace fallback scoping (#10176) (thanks @Yida-Dev)

* refactor: centralize workspace fallback classification and redaction (#10176) (thanks @Yida-Dev)

* test: remove explicit any from utils mock (#10176) (thanks @Yida-Dev)

* security: reject malformed agent session keys for workspace resolution (#10176) (thanks @Yida-Dev)

---------

Co-authored-by: Yida-Dev <reyifeijun@gmail.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Gustavo Madeira Santana <gumadeiras@gmail.com>

src/hooks/llm-slug-generator.ts

@@ -41,6 +41,7 @@ Reply with ONLY the slug, nothing else. Examples: "vendor-pitch", "api-design",
     const result = await runEmbeddedPiAgent({
       sessionId: `slug-generator-${Date.now()}`,
       sessionKey: "temp:slug-generator",
+      agentId,
       sessionFile: tempSessionFile,
       workspaceDir,
       agentDir,