ci: add build workflow

[phmatray]

Summary

• Adds .github/workflows/ci.yml running dotnet build -c Release on ubuntu-latest with .NET 10.0.x (actions/setup-dotnet@v4), matrixed over the solution paths that currently build green.
• Triggers on push to main and on pull requests targeting main.
• Only includes solutions verified to build successfully; the one that doesn't is excluded from the matrix and documented in a comment block at the top of the workflow, so it isn't silently dropped.
• No fixes to legacy/absorbed subproject code were made — that's explicitly out of scope for this change.

Build-health table

Every *.sln in the repo was built locally with dotnet build -c Release (.NET 10.0.301 SDK):

Solution	Status	Notes
devhub-sync/DevHubSync.sln	✅ builds	Clean build (a couple of pre-existing NU1903 vulnerable-package warnings, no errors). Included in CI matrix.
repo-tools/GitHubRepoTools.sln	❌ fails	GitHubRepoTools.Cli/Commands/EnableRenovateCommand.cs(58,37): error CS0507: EnableRenovateCommand.ExecuteAsync cannot change access modifiers when overriding the protected inherited member AsyncCommand<Settings>.ExecuteAsync. Pre-existing legacy code issue, excluded from CI matrix.

1/2 solutions build. skills/ and automate/ do not contain any .sln/.NET projects (Node/Tauri-based), so they aren't part of this build matrix.

Test plan

• dotnet build -c Release run locally against every discovered .sln (2 found), results captured above.
• Workflow YAML reviewed for correctness (checkout, setup-dotnet@v4 with dotnet-version: 10.0.x, matrix build step).
• CI will run automatically once this PR is opened/pushed — confirm the build (devhub-sync/DevHubSync.sln) job goes green.