releasekit sync: reconcile local version/tag state against registries #548

From the July 2026 direction review (§1.4). Convergent evolution across the field says this is a missing primitive: beachball has sync, lerna has publish from-package, release-plz diffs against registry-downloaded crates. releasekit's roll-forward model deserves a one-command recovery answer.

Scope

• releasekit sync: diff manifest/tag state against what each registry actually has (npm, crates.io, pub.dev — reusing the publish stage's existing version-existence checks).
• Report drift: published-but-untagged, tagged-but-unpublished, version mismatches, missing GitHub Releases.
• Repair mode: publish missing versions (idempotent, skip published), create missing tags/releases — the same primitives the publish stage already has, exposed as reconciliation.
• Dry-run by default; --apply to act. JSON output per the CLI contract epic.
• Complements release:retry and the failure-report comment; docs/troubleshooting.md gets a "recovering from partial publishes" pointer to it.