First release

Add NuGet publish workflow + shared package metadata

The publish job runs on `v*.*.*` tag pushes (or via manual workflow_dispatch
with a version input). It restores, builds Release with the resolved version
threaded through `-p:Version=`, runs unit tests as a release-gate sanity
check, packs all six src projects with symbol packages (.snupkg), and
pushes to nuget.org with --skip-duplicate so re-runs are idempotent.

Directory.Build.props now carries the shared NuGet metadata (Authors,
Repository*, license expression, readme, source-link config). Each
.csproj keeps its Description / RootNamespace; Cel.Parser was missing
both, so add them.

Local pack smoke-tested with -p:Version=0.1.0-dev — produces six nupkgs
+ snupkgs with the expected inter-package dependencies (Cel depends on
Cel.Checker, Cel.Core, Cel.Parser, Cel.Runtime).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>