feat: implement account management for multi-account Claude Code (gt-3133)

Adds support for managing multiple Claude Code accounts in Gas Town:

- accounts.json config parsing in mayor/ directory
- gt account list/add/default commands
- GT_ACCOUNT env var support with priority resolution
- --account flag on gt spawn and gt crew at commands
- CLAUDE_CONFIG_DIR injection into tmux sessions

Priority order: GT_ACCOUNT env var > --account flag > default from config

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

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

internal/cmd/crew.go

@@ -12,6 +12,7 @@ var (
 	crewForce   bool
 	crewNoTmux  bool
 	crewMessage string
+	crewAccount string
 )
 
 var crewCmd = &cobra.Command{
@@ -197,6 +198,7 @@ func init() {
 
 	crewAtCmd.Flags().StringVar(&crewRig, "rig", "", "Rig to use")
 	crewAtCmd.Flags().BoolVar(&crewNoTmux, "no-tmux", false, "Just print directory path")
+	crewAtCmd.Flags().StringVar(&crewAccount, "account", "", "Claude Code account handle to use (overrides default)")
 
 	crewRemoveCmd.Flags().StringVar(&crewRig, "rig", "", "Rig to use")
 	crewRemoveCmd.Flags().BoolVar(&crewForce, "force", false, "Force remove (skip safety checks)")