Show workflow activity in sidebar

[ymichael]

Summary

• Add thread list activity counts for active background workflows and background subagents.
• Show the workflow icon in sidebar status slots when the runtime is idle but a workflow is still running; keep the spinner for active runtime work.
• Invalidate thread-list queries when background-task activity starts or settles.
• Use a scoped CTE query for activity counts so the sidebar path does not scan all events or do per-row correlated latest-state lookups.

Validation

• pnpm exec turbo run typecheck --filter=@bb/app --filter=@bb/server --filter=@bb/db --filter=@bb/domain --filter=@bb/server-contract
• pnpm exec turbo run test --filter=@bb/db
• pnpm exec turbo run test --filter=@bb/app
• git diff --check

Perf notes

Measured against the local SQLite DB on this machine using a snapshot with ~148k events, 185 live threads, 12 visible sidebar threads, and 45 background-task events.

• Before branch, sidebar thread list + latest-session lookup: ~0.10 ms p50.
• Initial activity-query shape: ~3.6 ms p50, ~6.1 ms p95.
• Optimized activity query in this PR: ~0.23 ms p50, ~0.53 ms p95 for visible sidebar threads.
• Combined sidebar path after optimization: ~0.41 ms p50.

A live DB read while the app was writing measured ~0.38 ms p50, ~3.7 ms p95, and ~7.6 ms p99 for the optimized activity query.