feat: CoeditingRegistry opt-in gate (INV-10, spec §6.4) — enter/stop commands + context keys

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

package.json

@@ -118,6 +118,16 @@
         "command": "cowriting.proposalRejectMenu",
         "title": "Reject Claude Proposal",
         "category": "Cowriting"
+      },
+      {
+        "command": "cowriting.coeditDocument",
+        "title": "✦ Coedit this Document with Claude",
+        "category": "Cowriting"
+      },
+      {
+        "command": "cowriting.stopCoediting",
+        "title": "Stop editing with Claude",
+        "category": "Cowriting"
       }
     ],
     "menus": {
@@ -207,6 +217,16 @@
           "command": "cowriting.createThread",
           "when": "editorHasSelection && (resourceScheme == file || resourceScheme == untitled)",
           "group": "1_cowriting@2"
+        },
+        {
+          "command": "cowriting.coeditDocument",
+          "when": "editorLangId == markdown && !cowriting.isCoediting",
+          "group": "1_cowriting@1"
+        },
+        {
+          "command": "cowriting.stopCoediting",
+          "when": "editorLangId == markdown && cowriting.isCoediting",
+          "group": "1_cowriting@9"
         }
       ],
       "comments/commentThread/context": [