feat(lathe): add generated CLI self update

[samzong]

Summary

• Add root --version and -v support for generated Lathe CLIs using the existing build-time version values.
• Add optional update.github manifest config and a generated root update command backed by GitHub Releases.
• Reserve update as a root command and document version/update configuration in CLI usage docs.
• Update generated Skill guidance for CLI version and configured update maintenance commands.

Verification

• make check

Compatibility

Generated CLIs now expose root --version and -v; the existing hidden __lathe version maintenance command continues to use the same build-time version values. CLIs with update.github configured gain a root update command and generated Skill guidance for safe maintenance usage; flat generated API commands named update remain namespaced because update is now reserved. Catalog schema, auth, request body, and output formatting behavior are unchanged.

Checklist

• Tests or focused verification cover the changed surface.
• User-facing behavior changes are documented.
• Generated output under internal/generated/, .cache/, and ad-hoc skills/<cli-name>/ directories is not committed.
• Commits are signed off when this is ready to merge.