Forward initial image/file attachments when spawning ACP subagents through the existing sessions_spawn attachment opt-in. Remove the PR-only acpEnabled config split so ACP uses the same attachment gate as other runtimes.
Also fix the PR branch CI fallout: type the browser element CLI request mock and use Vitest env stubs in the Azure speech test to satisfy the changed-path security scan.
Verification:
- GitHub CI passed on f6ca26b160.
- Autoreview clean.
- Crabbox AWS live OpenAI proof passed: cbx_a576d49493fe / run_081dcc6c6a1b.
Thanks @zhangguiping-xydt.