feat: Add 'spawn' as alias for 'start' subcommand across roles

Added 'spawn' as an alias for 'start' in the following commands:
- gt witness start → also gt witness spawn
- gt refinery start → also gt refinery spawn
- gt deacon start → also gt deacon spawn
- gt crew start → also gt crew spawn

This improves discoverability since agents are guessing 'spawn'
when trying to start roles.

Note: gt polecat does not have a 'start' command - polecat spawning
is handled via 'gt sling'.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/cmd/deacon.go

@@ -39,8 +39,9 @@ and check status.`,
 }
 
 var deaconStartCmd = &cobra.Command{
-	Use:   "start",
-	Short: "Start the Deacon session",
+	Use:     "start",
+	Aliases: []string{"spawn"},
+	Short:   "Start the Deacon session",
 	Long: `Start the Deacon tmux session.
 
 Creates a new detached tmux session for the Deacon and launches Claude.