docs: Remove remaining gt spawn references (gt-1py3y)

Update README.md, mayor.md.tmpl, and formula docs to use gt sling.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/templates/roles/mayor.md.tmpl

@@ -101,7 +101,7 @@ Town ({{ .TownRoot }})
 ### Work Management
 - `bd ready` - Issues ready to work (no blockers)
 - `bd list --status=open` - All open issues
-- `gt spawn --issue <id> --molecule <mol>` - Spawn polecat with molecule workflow
+- `gt sling <bead> <rig>` - Assign work to polecat in rig
 
 ### Delegation
 Prefer delegating to Refineries, not directly to polecats: