Prefix changelog versions with v and add solo mode
Change generate_changelog_section to emit a 'v' before version numbers.
Treat solo workspaces specially in apply_release_plan: for
single-package
repos generate a per-package changelog section (version H2, no package
prefix) instead of aggregating into a dated global section.
Add create_solo_workspace helper and tests for solo changelog
formatting;
update megatest expectations accordingly.