Stabilize session event tests

[kitlangton]

Summary

• Replaces fixed 100ms sleeps in test/session/session.test.ts with waits resolved by actual event callbacks.
• Converts the file to the shared testEffect(...) pattern with direct Session.Service usage, it.instance(...), it.live(...), tmpdirScoped(...), and provideInstance(...).
• Uses GlobalBus for event assertions because SyncEvent still publishes through the legacy top-level Bus facade internally; that is the actual boundary these session events currently cross.
• Skips the legacy session.updated ordering assertion when experimental workspaces are enabled, because Session.create intentionally does not publish that legacy event in that mode.

Sleep Audit

• The session test had three fixed waits after bus events; all are now event-driven.
• A repo-wide audit shows many remaining sleeps are either explicit timeout guards, fixture worker delays, polling harnesses, or broader integration timing. I kept this PR focused on the locally reproduced session event flake.

Verification

• bun run test -- test/session/session.test.ts
• Repeated test/session/session.test.ts 5 times in a row
• bun typecheck in packages/opencode
• bunx prettier --check packages/opencode/test/session/session.test.ts
• bunx oxlint packages/opencode/test/session/session.test.ts (warnings only, existing patterns)
• git diff --check
• pre-push bun turbo typecheck