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>

CLAUDE.md

@@ -65,7 +65,7 @@ gastown/                    ← This rig
 - `gt rigs` - List all rigs
 
 ### Work Management
-- `gt spawn --issue <id>` - Spawn polecat for issue
+- `gt sling <bead> <rig>` - Assign work to polecat in rig
 - `bd update <id> --status=in_progress` - Claim work
 
 ## Development