Documentation Check

This PR adds a user-facing AI Budget Limit Reached notification (100% threshold), lowers the warning threshold from 90% to 85%, rewrites the notification body text, and makes the AI Budget notification group visible in user settings. These are user-facing behaviors, but the docs don't cover budget notifications.

Updates Needed

• docs/ai-coder/agents/platform-controls/usage-insights.md - The Spend Management page documents limit enforcement (409 blocking) but not the notifications users receive. Document that users are notified when their spend crosses the warning threshold (85%) and when they reach the limit (100%, after which requests are blocked).

New Documentation Needed

• docs/admin/monitoring/notifications/index.md - The notification events list has no AI Budget section, even though this PR adds a new user-facing notification template (AI Budget Limit Reached) and surfaces the AI Budget group in the notification settings UI. Add an "AI Budget Events" entry covering the warning and limit-reached notifications sent to affected users.

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