fix: queue system notices during active turns, and skip storing hidden ACP diagnostics

[timigod]

Fixes #5617

What changed

• A system notice (child completion, wait resumption) that reaches a thread with an active provider turn now goes into the durable thread queue and is delivered at the turn boundary. It no longer takes the live-steer path, so it can't cancel a running command or permission request. Human steers are unchanged.
• With diagnostic events hidden, ordinary ACP provider/unhandled events are no longer stored. Provider errors and terminal tool events (completed, failed, cancelled) are still stored.

Tests

• New: "parks a parent system notice without steering an active turn" (apps/server/test/threads/thread-send-dispatch.test.ts) and "can omit unhandled provider diagnostics without shifting stored sequences" (packages/db/test/data/events.test.ts).
• Full @bb/server suite on 075826d6 and on this branch, same environment: 3,502 passed, 3 failed, 1 skipped in both. The same three test files fail on 075826d6: the machine-install HOME cases, the marketplace committer-date case, and a missing plugin-authoring fixture.
• @bb/server typecheck passes.

Investigated, implemented and verified end to end by Timi's coding agents: gpt-6.1-sol at high reasoning level, coordinated by Claude Opus 5.5 at max thinking level.

🤖 Generated with Claude Code

AGENT GENERATED

[github-actions]

Thanks @timigod for the PR. This repo approves new contributors before it reviews their PRs, so this PR now closes on its own. That is expected. Join the #contributors channel in Discord (https://discord.gg/kvBU6tJhcJ), post a link to this PR, and explain what it changes and why. A maintainer then adds you to the allow list and can reopen the PR.