Fix AppEvents startup inbox readiness gating - #247

Summary

• Gates startup inbox processing in AppEvents.LoadAsync() so the hooked-events path waits for the readiness hookup that populates OlInboxes.
• Runs startup inbox processing from the readiness hookup path after inbox subscriptions are established.
• Adds focused regression coverage for pre-readiness suppression and post-hookup processing order.

Verification

• dotnet tool run csharpier format .
• Analyzer build passed with 0 errors.
• Nullable warnings-as-errors build passed with 0 warnings and 0 errors.
• Focused affected tests passed: 16/16.
• Full TaskMaster.Test passed: 198/198.
• Full coverage run passed: 4,972/4,972 tests.
• Corrected repository coverage: 79.9920%; baseline: 79.9234%; changed executable line coverage: 100.0000%.

Exception

• Repository-wide coverage is 0.0080 percentage points below the 80.0000% threshold.
• This threshold exception was explicitly authorized and is recorded in docs/features/active/2026-07-06-appevents-loadasync-inbox-gating-243/runbooks/coverage-threshold-exception.runbook.md.

Closes #243