Add Copilot skill: reproduce CI runs locally for dotnet/android lanes #11137

Problem

When a CI lane fails (e.g. CoreCLRTrimmable, NativeAOT, or a specific device test suite), an agent/contributor trying to reproduce it locally has to reverse-engineer:

• Which dotnet-local.sh target to invoke
• Which MSBuild properties to pass (_AndroidTypeMapImplementation, UseMonoRuntime, RuntimeIdentifier, etc.)
• Which configuration to use (Debug vs Release)
• How to run the device test (with emulator attached) vs the host test
• Where to find the results XML

This burned a significant amount of time during PR #11091 and forced CI-driven iteration (hours per cycle) instead of local iteration (minutes per cycle).

Proposal

Add a .github/skills/reproduce-ci-run/SKILL.md that maps a CI lane name to the exact local commands. Sketch:

Given: CoreCLRTrimmable lane failure
Produce:
  make prepare && make all CONFIGURATION=Release
  ./dotnet-local.sh build tests/Mono.Android-Tests/Mono.Android-Tests/Mono.Android.NET-Tests.csproj \
      -t:RunTestApp -c Release \
      -p:_AndroidTypeMapImplementation=trimmable \
      -p:UseMonoRuntime=false
  Parse TestResult-Mono.Android.NET_Tests-ReleaseCoreCLRTrimmable.xml

Coverage should include at minimum:

• Mono vs CoreCLR vs NativeAOT
• legacy vs trimmable typemap
• Debug vs Release
• Device tests (emulator/phone attached) vs host tests
• Build-only smoke vs full test matrix

Acceptance criteria

• Skill exists with frontmatter (name, description) and is triggered by phrases like "reproduce CI locally", "run CoreCLRTrimmable tests locally".
• Covers the lane/configuration combinations listed above.
• Documents where results land and how to parse them.
• Validated by reproducing at least one recent CI failure end-to-end.

Discovered during

PR #11091 — adding the CoreCLRTrimmable CI lane. See also companion issue about investigation/debugging practices (already added to .github/copilot-instructions.md).