Build Perf Baseline skill: what it does and how to install it

What it does. Establishes cold, warm and no-op baselines with dotnet build measurements and binary logs, then records before-and-after timings. Guides optimisation through artifacts output, deterministic builds, dependency-graph trimming, static graph builds, parallelism, separate restore and project filtering.

When to use it. Use before investigating or optimising slow .NET MSBuild builds, especially across multi-project solutions and CI. It does not cover non-MSBuild systems or detailed bottleneck analysis, which belongs in the diagnostics skill.