config(fish): add fish completions for cli tools

dot_config/fish/functions/ai.fish

@@ -0,0 +1,13 @@
+function ai --description 'Choose and launch an AI CLI via gum'
+    set -l choice (gum choose --header 'AI CLI' claude opencode codex hermes fractal)
+    test -n "$choice"; or return  # cancelled (Esc / Ctrl-C) → bail out
+
+    switch $choice
+        case claude
+            claude --dangerously-skip-permissions
+        case '*'
+            # labels match the binary names, so just run it
+            # add a dedicated `case` here if another tool needs flags
+            $choice
+    end
+end