Versioning Phase 5 — native version computation in `bv`, drop `nbgv`

[rdeago]

Sub-issue of #267. Depends on Phase 1 (#268) and Phase 4 (#271).

Scope

• GitService gains height via LibGit2Sharp: commits from HEAD back to (but not including) the last commit that modified current-version.json. Reuse the existing short CurrentBranch.
• Rewrite VersionService to compute from GitService + Buildvana.Versioning + BuildvanaConfig (release.branches, release.prereleaseTag); delete the nbgv subprocess and JSON-parsing path. Add release.branches/release.prereleaseTag to the config model + schema here (first consumer); enforce "version is prerelease but prereleaseTag absent ⇒ fail the release."
• Switch VersionFile read/write to current-version.json. Remove the nbgv entry from .config/dotnet-tools.json.
• The repo gains its own current-version.json + the relevant buildvana.json keys; version.json is retained only for the still-NBGV-based SDK build until Phase 6 publishes.

Acceptance criteria

• VersionService no longer references nbgv; .config/dotnet-tools.json no longer lists it.
• Height increases monotonically with commits within a version line and resets at each current-version.json change; the overall version never regresses.
• A prerelease current-version.json with no release.prereleaseTag fails the release with a clear message.
• dotnet bv release (dry run) and version-consuming commands work without nbgv.
• CHANGELOG: Unreleased entries; **BREAKING CHANGE** for the version.json → current-version.json migration.

[rdeago]

Superseded by #293/#294/#296, see #267.