Versioning Phase 8 — `bv version` command

[rdeago]

Sub-issue of #267. Depends on Phase 7 (#296).

Scope

• Spectre subcommands bv version show and bv version advance <kind>.
  • show: current version, latest, latest stable, IsPublicRelease, IsPrerelease, current branch.
  • advance: load current-version.json, apply ApplyVersionSpecChange(kind), save; --commit (off by default) creates a commit, otherwise leaves the change for manual review.
• Reuse existing VersionFile.ApplyVersionSpecChange / VersionService.ComputeVersionSpecChange; help text consistent with the rest of bv.

Acceptance criteria

• bv version show prints current version info without nbgv; bv version advance minor rewrites current-version.json and a subsequent show reflects it.
• --commit produces a well-formed commit; without it, the working tree is left modified.
• No regression in bv release.
• CHANGELOG: Unreleased entry for the new bv version command.