refactor(agents): scope customization to monorepo

Move Pi runtime resources into project-local .pi directories and keep shell modules responsible for installing Pi, Herdr, gitea-axi, and Pi package patches.

Remove legacy global agent context, Claude Code customization, the global skills bridge, and the external skills flake input.

configs/nixos/modules/shell/gitea-axi.nix

@@ -0,0 +1,20 @@
+{
+  config,
+  lib,
+  inputs,
+  ...
+}:
+# gitea-axi for the primary user, installed through its own home-manager module.
+let
+  cfg = config.modules.shell.gitea-axi;
+  user = config.user.name;
+in
+{
+  options.modules.shell.gitea-axi.enable =
+    lib.mkEnableOption "gitea-axi, an agent-ergonomic CLI for Gitea issues and pull requests";
+
+  config = lib.mkIf cfg.enable {
+    home-manager.sharedModules = [ inputs.gitea-axi.homeModules.default ];
+    home-manager.users.${user}.programs.gitea-axi.enable = true;
+  };
+}