chore(release): 0.15.0 (#101)

Bump all sixteen workspace packages to 0.15.0 and add the consolidated
CHANGELOG entry. Highlights: two new published plugins (repo-auth,
lightbridge), the two shared-core libraries they compose (auth-core,
core-otel), and ADR-0012.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>