[Feature] Add cancel_task tool for stopping running background tasks

[randomm]

Summary

The Project Manager agent can dispatch tasks via mcp_task and check status via mcp_check_task, but cannot cancel running tasks. This limits orchestration capability.

Problem

When requirements change or a better-scoped task is dispatched, old tasks continue running:

• Wastes compute/tokens
• May produce conflicting changes
• No way to stop runaway or stuck tasks

Current State

• ✅ mcp_task - dispatch tasks to specialists
• ✅ mcp_check_task - check task status
• ❌ cancel_task - missing

Requirements

1. Add cancel_task tool that accepts task_id
2. Should gracefully stop the running agent
3. Return confirmation: { "status": "cancelled", "task_id": "..." }
4. Handle edge cases: task already completed, task not found

Implementation

• Find task management code (likely packages/opencode/src/tool/task.ts)
• Add AbortController tracking per task
• Implement cancel_task tool
• Register tool in MCP tool registry
• Add tests

Acceptance Criteria

• cancel_task tool implemented
• Cancelled tasks stop execution promptly
• Status returns "cancelled" when checked
• Works for all agent types

[randomm]

Completed in commit 1fd1128. cancel_task tool aborts subagent sessions correctly.