Reduce trace2 event nesting

[svarlamov]

Summary

• lower the installed and diagnosed trace2.eventNesting value from 10 to 0
• update trace2-backed test harness defaults and direct raw Git trace invocations to exercise nesting 0 by default
• keep the override hook GIT_AI_TEST_TRACE2_NESTING available for targeted comparisons
• restack this on Fix Windows CI flakes #1739, which fixes the Windows CI failure already present on main

Validation

• GIT_AI_TEST_TRACE2_NESTING=0 task test CARGO_TEST_ARGS='--test commit_tree_update_ref'
• GIT_AI_TEST_TRACE2_NESTING=0 task test
• task fmt
• task test CARGO_TEST_ARGS='--test commit_tree_update_ref'
• task test CARGO_TEST_ARGS='--test async_mode'
• task test TEST_FILTER=graphite
• task test CARGO_TEST_ARGS="--test notes_sync_regression"
• task test CARGO_TEST_ARGS="--test integration" TEST_FILTER=tls_native_certs::test_https_request_uses_system_certs
• task lint
• git diff --check
• task test with local /home/ubuntu/.opencode/bin removed from PATH because this workstation has an unrelated opencode binary that makes mdm::agents::opencode::tests::test_opencode_no_binary_no_config_not_detected fail on main too

---

[svarlamov]

• Reduce trace2 event nesting #1738 👈 (View in Graphite)
• Fix Windows CI flakes #1739
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

[Siddhant-K-code]

@svarlamov - I am unsure if it is intentional to have duplicate work from #1735 ?