fix(executor): support attached-mode opencode prompt flow

[myagizmaktav]

Summary

• send attached-mode directory through query params and headers during session create, prompt, event subscribe, and permission replies
• support modern OpenCode attached-mode execution via prompt_async + /event with fallback to legacy /message flow
• parse both wrapped and bare OpenCode event payloads and auto-approve matching external directory permissions

Verification

• go test ./internal/executor -run TestOpenCodeBackend -count=1
• go test ./internal/executor -count=1
• manual local pilot task run advanced past Initializing opencode... and completed

Closes #2415
Also addresses #2407 for attached-mode servers on OpenCode 1.4.6+