ci(tests[benchmarks]) Add CodSpeed performance benchmarks

[tony]

Summary

• Adds a CodSpeed-backed CI benchmark job for tracking GitSync.obtain (and future hot paths) performance over time
• Functional-style benchmark test in tests/benchmarks/ (no class TestX: patterns)
• Benchmarks are deselected by default via the benchmark marker; opt-in with pytest tests/benchmarks --codspeed -m benchmark
• Replaces the long-stalled poetry-era Codspeed - performance benchmarks #471 with a uv-native implementation against current master

Changes

• pyproject.toml
  • Add pytest-codspeed to the testing dependency group
  • Register the benchmark marker
  • Default addopts adds -m 'not benchmark' so regular uv run py.test skips benchmarks
  • mypy override to ignore missing pytest_codspeed.* stubs
• tests/benchmarks/test_sync_git.py — single representative benchmark for GitSync.obtain against an initial-commit remote (~49ms locally)
• .github/workflows/tests.yml — new benchmarks job running CodSpeedHQ/action@v3 on pull requests and master pushes
• CHANGES — entry under 0.41.x (unreleased)

Test plan

• uv run ruff check . — clean
• uv run ruff format . — clean
• uv run mypy — clean
• uv run py.test --reruns 0 -vvv — 615 passed, 1 skipped, 1 deselected (the new benchmark, as designed)
• just build-docs — succeeds
• Local benchmark run (uv run --group testing py.test tests/benchmarks --codspeed -m benchmark) — test_git_obtain_initial_commit_repo 48.8ms median
• CI `benchmarks` job runs on PR (requires `CODSPEED_TOKEN` repo secret)

Supersedes #471 (which targeted the poetry-era pyproject.toml; closed).

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.23%. Comparing base (d9391c3) to head (90af158).

Files with missing lines	Patch %	Lines
tests/benchmarks/test_sync_git.py	60.00%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #525   +/-   ##
=======================================
  Coverage   58.23%   58.23%           
=======================================
  Files          40       41    +1     
  Lines        6489     6499   +10     
  Branches     1098     1098           
=======================================
+ Hits         3779     3785    +6     
- Misses       2177     2181    +4     
  Partials      533      533           

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