feat(dev): add gastown multi-agent workspace manager

- Add gastown flake input (non-flake, source only)
- Build gastown package using buildGoModule in development role
- Configure renovate for daily updates of gastown and beads
- Binary: gt (Gas Town CLI by Steve Yegge)

flake.nix

@@ -47,6 +47,11 @@
       inputs.nixpkgs.follows = "nixpkgs-unstable";
     };
 
+    gastown = {
+      url = "github:steveyegge/gastown";
+      flake = false;  # No flake.nix upstream yet
+    };
+
     nix-doom-emacs-unstraightened = {
       url = "github:marienz/nix-doom-emacs-unstraightened";
       # Don't follow nixpkgs to avoid rebuild issues with emacs-overlay