F10 SLICE-1: retire dead attribution toggle (visible/toggle/isVisible + command)

Code-quality follow-up: with editor decorations gone (Task 1), the visibility
toggle was dead/misleading — nothing reads `visible`. Remove the field,
toggle()/isVisible(), the cowriting.toggleAttribution registration, and its
package.json command + palette declarations (spec §6.2 retires it). renderActive/
render stay — they refresh the orphan status bar on editor switch.
E2E toggle assertions (attribution.test.ts, noWorkspace.test.ts) are updated in SLICE-4.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

package.json

@@ -44,11 +44,6 @@
         "title": "Reopen Thread",
         "category": "Cowriting"
       },
-      {
-        "command": "cowriting.toggleAttribution",
-        "title": "Toggle Attribution",
-        "category": "Cowriting"
-      },
       {
         "command": "cowriting.applyAgentEdit",
         "title": "Apply Agent Edit (internal seam)",
@@ -115,10 +110,6 @@
         {
           "command": "cowriting.pinDiffBaseline",
           "when": "false"
-        },
-        {
-          "command": "cowriting.toggleAttribution",
-          "when": "false"
         }
       ],
       "editor/context": [