Add gt ss and gt undo commands (#180)

Both are common graphite commands. Descriptions adapted from [https://graphite.dev/docs/command-reference](https://graphite.dev/docs/cheatsheet)

Adding `gt ss` is very useful since previously this would autocorrect to `gt squash`:

![image.png](https://app.graphite.dev/user-attachments/assets/f6f18b40-af08-4eef-94c6-f1bfb8164e63.png)

Note that `gt ss` is a default alias which can be overriden, but only by assigning something else to the `ss` alias, in which case we would still want to stop this from auto-correcting.

Author: smchase

command-signatures/json/gt.json

@@ -586,6 +586,20 @@
           "args": { "name": "steps", "default": "1" }
         }
       ]
+    },
+    {
+      "name": "ss",
+      "description": "Default alias for 'submit --stack', submit your changes across all PRs on a stack"
+    },
+    {
+      "name": "undo",
+      "description": "Undo the most recent Graphite mutations",
+      "options": [
+        {
+          "name": ["--force", "-f"],
+          "description": "Do not prompt for confirmation; undo the most recent command immediately"
+        }
+      ]
     }
   ],
   "options": [