feat: Remove gt spawn completely - gt sling is THE command (gt-1py3y)

Fully remove gt spawn from the codebase:

- Delete spawn.go, create polecat_spawn.go with just sling helpers
- Remove all gt spawn references from docs and CLAUDE.md
- Update code comments to reference gt sling

gt sling now handles ALL work dispatch:
- Existing agents: gt sling <bead> mayor/crew/witness
- Auto-spawn: gt sling <bead> <rig>
- No-tmux: gt sling <bead> <rig> --naked
- With args: gt sling <bead> --args "..."

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

docs/bootstrap.md

@@ -221,4 +221,4 @@ echo "Bootstrap complete!"
 After bootstrapping:
 1. Start a Mayor session: `gt mayor attach`
 2. Check for work: `bd ready`
-3. Spawn workers with molecules: `gt spawn --issue <id> --molecule mol-shiny`
+3. Spawn workers with molecules: `gt sling <id> <rig> --molecule mol-shiny`