Show background process progress (similar to /agents)

Feature request

Add a way to see live progress for background processes, similar to how /agents shows what subagents are doing.

Problem

When a background task runs (builds/tests/indexing/analysis), there is no visibility into what step it is on, whether it is progressing, or how long it has been running. This makes it hard to trust long-running jobs and decide whether to wait, cancel, or re-run.

Proposed solution

Provide a background-process view/panel/command that shows:

• current step/phase
• recent log tail (last N lines)
• start time + elapsed time
• percent/estimated completion when available
• ability to open details or follow logs live

The UX could mirror the existing /agents activity view for consistency.

Acceptance criteria

• Users can list active background processes
• Each process shows live status/step and elapsed time
• Users can drill into logs and follow progress
• Works for common long-running tasks (build/test/indexing/etc.)