Add rbw SSH socket support in home-manager modules

- Introduced a new module `rbw-ssh-socket.nix` to manage SSH_AUTH_SOCK for rbw.
- Updated `default.nix` to utilize the new SSH socket variable for session management.
- Refactored Bash and Fish shell configurations to use the new module for setting SSH_AUTH_SOCK, improving maintainability and consistency across shell environments.

modules/lib/rbw-ssh-socket.nix

@@ -0,0 +1,30 @@
+{ ... }: {
+  flake.lib.rbwSshSocket =
+    let
+      bashSnippet = ''
+        if [ -z "''${SSH_AUTH_SOCK:-}" ]; then
+          if [ -n "''${XDG_RUNTIME_DIR:-}" ]; then
+            export SSH_AUTH_SOCK="$XDG_RUNTIME_DIR/rbw/ssh-agent-socket"
+          else
+            export SSH_AUTH_SOCK="/run/user/$(id -u)/rbw/ssh-agent-socket"
+          fi
+        fi
+      '';
+      fishSnippet =
+        pkgs: ''
+          if test -z "$SSH_AUTH_SOCK"
+            if test -n "$XDG_RUNTIME_DIR"
+              set -gx SSH_AUTH_SOCK "$XDG_RUNTIME_DIR/rbw/ssh-agent-socket"
+            else
+              set -l _hm_uid (${pkgs.coreutils}/bin/id -u)
+              set -gx SSH_AUTH_SOCK "/run/user/$_hm_uid/rbw/ssh-agent-socket"
+            end
+          end
+        '';
+    in
+    {
+      inherit bashSnippet fishSnippet;
+      sessionVariable = "$XDG_RUNTIME_DIR/rbw/ssh-agent-socket";
+      activationPath = "/run/user/$(id -u)/rbw/ssh-agent-socket";
+    };
+}