Harden Syncwheel housekeeping guidance

[Yehonal]

Summary

• harden the Syncwheel skill with a managed-repo detection reflex and post-merge housekeeping guidance
• document squash/rebase merge verification by content, plus cleanup of worktrees, branches, and backup refs
• add a design spec for a future automated housekeep command, hooks, and CI triggers
• bump package metadata to 0.20.1 and note the change in the changelog

Validation

• python3 scripts/check-version-bump.py --staged
• syncwheel validate (warned only that local main-integration is missing)