Notification hook should fire when agent is blocked waiting for tool approval (VSCode extension)

Feature Request

When running multiple agents in parallel in the VSCode extension, there's no way to
know when one has paused waiting for tool approval. The Notification hook exists
but does not fire in this scenario — approvals appear inline in the Claude panel with
no external signal.

Proposed behavior

Fire the Notification hook event when an agent is blocked waiting for user approval
on a tool call. Ideally include agent context in the payload so the notification can
identify which agent needs attention.

Current workaround

None effective for multi-agent workflows. The Notification hook works for
headless/terminal use but not for the interactive VSCode extension approval flow.

Why it matters

With multiple parallel agents, users have no visibility into which agent is waiting
without actively watching the panel. A hook event would allow desktop notifications
(e.g. via terminal-notifier on macOS) to surface this automatically.