Add tea 0.10.1 nix derivation and Gitea PR skill

- Pin tea CLI to 0.10.1 to avoid TTY bug in 0.11.x
- Add .claude/skills/gitea for PR creation workflow
- Document tea CLI usage in CLAUDE.md

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

devenv.nix

@@ -1,5 +1,10 @@
 { pkgs, ... }:
 
+let
+  # Use tea 0.10.1 to avoid TTY bug in 0.11.x
+  # See: https://gitea.com/gitea/tea/issues/827
+  tea = pkgs.callPackage ./nix/tea.nix { };
+in
 {
   packages = with pkgs; [
     nixfmt-rfc-style