feat(f6): contribute toggleDiffView + pinDiffBaseline commands + ctrl+alt+d (SLICE-3)

F6 §5/§6.4. Palette-visible commands; ctrl+alt+d (when editorTextFocus),
unbound in stock VS Code, user-remappable.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

package.json

@@ -73,6 +73,16 @@
         "command": "cowriting.proposeAgentEdit",
         "title": "Propose Agent Edit (internal seam)",
         "category": "Cowriting"
+      },
+      {
+        "command": "cowriting.toggleDiffView",
+        "title": "Cowriting: Toggle Diff View",
+        "category": "Cowriting"
+      },
+      {
+        "command": "cowriting.pinDiffBaseline",
+        "title": "Cowriting: Pin Diff Baseline to Now",
+        "category": "Cowriting"
       }
     ],
     "menus": {
@@ -135,7 +145,14 @@
           "when": "commentController == cowriting.proposals"
         }
       ]
-    }
+    },
+    "keybindings": [
+      {
+        "command": "cowriting.toggleDiffView",
+        "key": "ctrl+alt+d",
+        "when": "editorTextFocus"
+      }
+    ]
   },
   "scripts": {
     "build": "node esbuild.mjs",