Support empty changesets (chronus add --empty)

[timotheeguerin]

Summary

Support creating an "empty" changeset that documents intent to release without bumping any package, mirroring Changesets' changeset add --empty.

Motivation

Sometimes you need to force a release or record a changelog note that isn't tied to a package version bump, e.g.:

• Trigger a republish/CI release without a functional change.
• Land a placeholder to satisfy chronus verify when the change genuinely warrants no version bump but you still want an explicit, reviewed record.

Today chronus add always requires selecting packages and a change kind. There is no way to produce a no-op/empty entry.

Proposed behavior

• chronus add --empty creates a valid changeset file with no packages (or an explicit none version type) and an empty/placeholder message.
• chronus verify and chronus version treat it as satisfying documentation requirements without producing a version bump.
• Interactive add offers an "empty changeset" escape hatch.

References

• Changesets --empty flag
• packages/chronus/src/cli/commands/add-changeset.ts
• Existing internal change kind with versionType: none (related but still targets a package).