feat: native diff surface — QuickDiff + cowriting-baseline: provider + Review Changes + status bar (spec §6.4, INV-13)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

package.json

@@ -87,7 +87,14 @@
       {
         "command": "cowriting.markReviewed",
         "title": "Mark Changes as Reviewed",
-        "category": "Cowriting"
+        "category": "Cowriting",
+        "icon": "$(check-all)"
+      },
+      {
+        "command": "cowriting.reviewChanges",
+        "title": "Review Changes",
+        "category": "Cowriting",
+        "icon": "$(git-compare)"
       },
       {
         "command": "cowriting.showTrackChangesPreview",
@@ -182,6 +189,16 @@
         }
       ],
       "editor/title": [
+        {
+          "command": "cowriting.reviewChanges",
+          "when": "resourceLangId == markdown && cowriting.isCoediting",
+          "group": "navigation@1"
+        },
+        {
+          "command": "cowriting.markReviewed",
+          "when": "resourceLangId == markdown && cowriting.isCoediting && cowriting.baselineMode == snapshot",
+          "group": "navigation@4"
+        },
         {
           "command": "cowriting.showTrackChangesPreview",
           "when": "editorLangId == markdown",