feat: crew attach auto-detection, worktree polecats, beads mail

- gt crew at: auto-detect crew from cwd, run gt prime after launch
- Polecats now use git worktrees from refinery (faster than clones)
- Updated architecture.md for two-tier beads mail model
- Town beads (gm-*) for Mayor mail/coordination
- Rig .beads/ symlinks to refinery/rig/.beads

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

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

internal/cmd/spawn.go

@@ -81,7 +81,7 @@ func runSpawn(cmd *cobra.Command, args []string) error {
 		return fmt.Errorf("not in a Gas Town workspace: %w", err)
 	}
 
-	rigsConfigPath := filepath.Join(townRoot, "config", "rigs.json")
+	rigsConfigPath := filepath.Join(townRoot, "mayor", "rigs.json")
 	rigsConfig, err := config.LoadRigsConfig(rigsConfigPath)
 	if err != nil {
 		rigsConfig = &config.RigsConfig{Rigs: make(map[string]config.RigEntry)}