[Feature Request] Agent Teams: Add notification level control for idle and peer DM notifications

Problem

When using Agent Teams, the human operator receives frequent automatic notifications that don't require action:

• Idle notifications: Sent every time a teammate's turn ends
• Peer DM summaries: Sent when teammates message each other

For a typical implement workflow (developer + reviewer), this can generate 10-20+ notifications per feature, most of which are informational only.

Proposed Solution

Add a configurable notification level, e.g.:

• all (default): Current behavior
• minimal: Only show notifications that require human action (plan approval, task completion reports, direct questions)
• none: Suppress all teammate notifications

This could be configured via:

• settings.json: "teamNotificationLevel": "minimal"
• Per-team override in TeamCreate parameters
• Or a /team subcommand like /team notifications minimal

Context

Related issues:

• #24414 (idle spawn token cost)
• #24246 (idle status delay)