fix(nudge): throttle idle goal notification scans

[TZZheng]

Summary

• throttle IDLE goal-notification scans to once per 10 seconds with per-agent monotonic state
• preserve the post-refactor: extract notification store port #881 NotificationStore.snapshot(allow) boundary; no direct filesystem reads or persistent throttle state
• add deterministic fake-clock coverage for cadence expiry, clearing, dismissal/republication, and delete/reappear behavior
• update both relevant anatomy documents

Tests

• focused notification validation — 136 passed in 8.00s
• tests/test_goal_notification.py — 8 passed
• notification store, system notification, and notification tool suites — 70 passed
• git diff --check — passed

Goal-state changes can be observed up to 10 seconds later, by design.

Fixes #751