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/gitinit.go

@@ -29,7 +29,7 @@ This command:
   3. Optionally creates a GitHub repository
 
 The .gitignore excludes:
-  - Polecat worktrees and rig clones (recreated with 'gt spawn' or 'gt rig add')
+  - Polecat worktrees and rig clones (recreated with 'gt sling' or 'gt rig add')
   - Runtime state files (state.json, *.lock)
   - OS and editor files
 
@@ -64,7 +64,7 @@ const HQGitignore = `# Gas Town HQ .gitignore
 **/registry.json
 
 # =============================================================================
-# Rig git worktrees (recreate with 'gt spawn' or 'gt rig add')
+# Rig git worktrees (recreate with 'gt sling' or 'gt rig add')
 # =============================================================================
 
 # Polecats - worker worktrees