Agents cancelling sub-agent

### Describe the feature or problem you'd like to solve

Agents cannot cancel background sub-agents they launched — only users can, via the /tasks UI

### Proposed solution

A cancel tool taking an agent_id would mirror what the /tasks UI already gives users, letting agents actively manage delegated work — not just wait for it.

### Example prompts or workflows

1. An orchestrator runs parallel research agents; when one returns a definitive finding, the others become redundant — it should be able to cancel them rather than wait them out.
2. An orchestrator enforces a time budget: after 15 minutes without completion, cancel and re-dispatch with a more focused prompt.
3. A user correction arrives mid-flight; the orchestrator should abort in-progress sub-agents before acting on the new direction.

### Additional context

_No response_