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/deacon-plugins.md

@@ -267,7 +267,7 @@ Execute registered plugins whose gates are open.
 ## Limitations
 
 - **No polecat spawning**: Plugins cannot spawn polecats. If a plugin tries
-  to use `gt spawn`, behavior is undefined. This may change in the future.
+  to use `gt sling`, behavior is undefined. This may change in the future.
 
 - **No cross-plugin dependencies**: Plugins don't declare dependencies on
   each other. If ordering matters, mark both as `parallel: false`.