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>

internal/cmd/sling.go

@@ -72,7 +72,7 @@ var (
 	slingVars     []string // --var flag: formula variables (key=value)
 	slingArgs     string   // --args flag: natural language instructions for executor
 
-	// Flags migrated from gt spawn for unified work assignment
+	// Flags migrated for polecat spawning (used by sling for work assignment
 	slingNaked    bool   // --naked: no-tmux mode (skip session creation)
 	slingCreate   bool   // --create: create polecat if it doesn't exist
 	slingMolecule string // --molecule: workflow to instantiate on the bead