feat: Add list_tasks tool to query all background tasks

[randomm]

Summary

Currently, we have task and check_task tools, but no way to list all running/pending tasks. We need a list_tasks tool to get an overview of all background tasks.

Current State

• ✅ task tool - Spawn new agents (sync/async)
• ✅ check_task tool - Check individual task status by ID
• ❌ No way to list all tasks without knowing their IDs

Proposed Solution

Add a new list_tasks tool that exposes the existing internal listBackgroundTasks() function from packages/opencode/src/session/async-tasks.ts.

Use Cases

• Track multiple concurrent tasks
• Get overview of running/pending/completed tasks
• Find task IDs without having saved them
• Debug task scheduling issues

Implementation Notes

• Internal function listBackgroundTasks() already exists
• Returns { pending: string[], results: Record<string, BackgroundTaskResult> }
• Need to add tool wrapper with proper parameters and documentation
• Should filter to only show tasks accessible to the caller session

Acceptance Criteria

• list_tasks tool created
• Returns pending task IDs and completed results
• Properly filtered by session ownership
• Documentation added
• Tests added
• Fork-features manifest updated