Automate releases with release-please

[vaceslav]

Summary

Replace the manual release process with release-please GitHub Action to automate versioning, changelog generation, and GitHub Release creation.

Current Process (manual)

1. Decide version bump
2. Update <Version> in .csproj
3. Update <PackageReleaseNotes> in .csproj
4. Move changelog entries from [Unreleased] to versioned section
5. Commit chore: prepare release vX.Y.Z
6. Tag & create GitHub Release
7. NuGet publish triggers automatically

Proposed Change

Add release-please GitHub Action that:

• Reads Conventional Commits to determine version bump (feat: → minor, fix: → patch)
• Automatically creates/updates a "Release PR" with version bump and changelog
• On merge: creates git tag, GitHub Release, which triggers existing publish.yml

Benefits

• No manual version bumping or changelog editing
• Version is always consistent with commit history
• Existing NuGet publish workflow (publish.yml) remains unchanged
• Release PR serves as a review gate before publishing

Trade-offs

• Changelog is generated from commit messages (less curated, but editable in PR)
• <PackageReleaseNotes> in .csproj may need extra-files config