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/architecture.md

@@ -699,7 +699,7 @@ pending → in_progress → completed
 When a molecule is attached to an issue:
 
 ```bash
-gt spawn --issue gt-xyz --molecule mol-shiny
+gt sling gt-xyz <rig> --molecule mol-shiny
 ```
 
 1. Molecule is validated (steps, dependencies)
@@ -756,7 +756,7 @@ bd mol show mol-shiny
 bd mol bond mol-shiny --var feature_name="auth"
 
 # Spawn polecat with molecule
-gt spawn --issue gt-xyz --molecule mol-shiny
+gt sling gt-xyz <rig> --molecule mol-shiny
 ```
 
 ### Why Molecules?
@@ -1957,7 +1957,7 @@ gt capture <polecat> "<cmd>"     # Run command in polecat session
 ### Session Management
 
 ```bash
-gt spawn --issue <id> --molecule mol-shiny  # Spawn polecat with workflow
+gt sling <id> <rig> --molecule mol-shiny  # Spawn polecat with workflow
 gt handoff             # Polecat requests shutdown (run when done)
 gt session stop <p>    # Kill polecat session (Witness uses this)
 ```