Add Nerdbank.GitVersioning for git-derived version numbers

[StuartMeeks]

What

Implements the Nerdbank.GitVersioning (NBGV) carried-over item. The version string now maintains itself from git instead of falling back to the assembly's 1.0.0 compile-time default — so the About page shows a real version for dev and CI builds, and releases derive their number from NBGV rather than the hand-typed tag.

How it works

• version.json (repo root) is the single source of truth: base 0.1.0-alpha.{height}, where {height} is the git commit height. versionHeightOffset: 1 starts the counter at alpha.2, past the previously hand-tagged v0.1.0-alpha.1, so there's no tag collision.
• NBGV is referenced centrally (Directory.Packages.props + a PrivateAssets="all" reference in Directory.Build.props) so every assembly is stamped. Verified the Core assembly now carries AssemblyInformationalVersion = 0.1.0-alpha.2+<commit> — which is exactly what SettingsViewModel reads for the About page.
• release.yml now installs nbgv, derives the version from nbgv get-version (asserting PublicRelease per publicReleaseRefSpec), and drops the redundant -p:Version on publish (NBGV stamps it). On a tagged public build, SemVer2 is the clean 0.1.0-alpha.N fed to Velopack.

Release ritual change (documented in CONTRIBUTING.md)

The tag trigger is unchanged, but you now mint the tag with NBGV so its name matches the computed version:

nbgv tag                       # e.g. v0.1.0-alpha.7
git push origin v0.1.0-alpha.7

You no longer hand-pick the -alpha.N counter — it tracks git height. For a stable cut, edit version.json (e.g. 0.1.0-alpha.{height} → 0.1.0), commit, then nbgv tag.

Verification

• nbgv get-version post-commit → 0.1.0-alpha.2+f80913fbe3; Core assembly stamped accordingly (checked the generated AssemblyInfo/ThisAssembly).
• WinUI head compiles clean on the Windows build agent — 0 warnings, 0 errors — including the no-.git case (NBGV degrades gracefully; TreatWarningsAsErrors not tripped).
• ci.yml already checks out with fetch-depth: 0, so CI computes height correctly.

🤖 Generated with Claude Code