perf(api): read notifications without full workspace snapshot

[kvnloo]

Problem

GET /notifications currently calls service.snapshot(), which loads tasks, goals, monitors, ideas, memories, artifacts, notifications, identity, and worker state only to return the notifications array.

With retained terminal tasks, that makes notification polling scale with the entire workspace.

A downstream PGlite benchmark with 2,000 task rows measured roughly:

• full snapshot path: ~2264ms / 39.3MB
• direct notifications list: ~2.0ms / 4.7KB

Change

Read notifications directly from the Store. Payload shape and ordering stay the same as the existing db.list used by the snapshot.

Regression

The API test stubs AgentService.snapshot() to throw and proves GET /notifications still succeeds. This fails on the old route and passes with the scoped read.

No notification mutation, ownership, or UI behavior changes.