feat(cli): display instance admin status in organization describe

[Piskoo]

Adds instance admin status to the org describe command output. For table format, it displays (Instance admin). For JSON format, it adds an instanceAdmin boolean field to the currentUser object.

┌─────────────────────────────────────────────────────────────────────────────┐
│ Current Context                                                             │
├─────────────────────┬───────────────────────────────────────────────────────┤
│ Logged in as        │ sarah@chainloop.local (Instance admin)                │
├─────────────────────┼───────────────────────────────────────────────────────┤
│ Organization        │ myorg (role=owner)                                    │
│                     │ Policy strategy=ADVISORY                              │
├─────────────────────┼───────────────────────────────────────────────────────┤
│ Default CAS Backend │ index.docker.io/piskop (provider=OCI, status="valid") │
└─────────────────────┴───────────────────────────────────────────────────────┘

{
   "currentUser": {
      "id": "34030166-826c-4c72-a635-353230fedb47",
      "email": "sarah@chainloop.local",
      "firstName": "",
      "lastName": "",
      "createdAt": "2025-06-12T10:39:01.096625Z",
      "instanceAdmin": true
   },
   "currentMembership": {
      "id": "c7740e0e-1a0c-467b-8bde-036b4d753bf7",
      "current": true,
      "joinedAt": "2025-06-16T09:09:23.352928Z",
      "updatedAt": "2025-06-16T09:09:23.352928Z",
      "org": {
...

Closes #2724