When polecat branches have .beads/ tracked from previous bd sync operations,
those files would contaminate the new worktree. Now setupSharedBeads():
1. Removes any existing .beads/ contents before creating redirect
2. Points redirect directly to mayor/rig/.beads (matches crew workers)
Also adds sling-design.md documenting the gt sling command design.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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