feat(foundry): add manual task owner change via UI dropdown

Add an owner dropdown to the Overview tab that lets users reassign task
ownership to any organization member. The owner's GitHub credentials are
used for git operations in the sandbox.

Full-stack implementation:
- Backend: changeTaskOwnerManually action on task actor, routed through
  org actor's changeWorkspaceTaskOwner action, with primaryUser schema
  columns on both task and org index tables
- Client: changeOwner method on workspace client (mock + remote)
- Frontend: owner dropdown in right sidebar Overview tab showing org
  members, with avatar and role display
- Shared: TaskWorkspaceChangeOwnerInput type and primaryUser fields on
  workspace snapshot types

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

foundry/packages/client/test/subscription-manager.test.ts

@@ -77,6 +77,8 @@ function organizationSnapshot(): OrganizationSummarySnapshot {
         pullRequest: null,
         activeSessionId: null,
         sessionsSummary: [],
+        primaryUserLogin: null,
+        primaryUserAvatarUrl: null,
       },
     ],
   };
@@ -159,6 +161,8 @@ describe("RemoteSubscriptionManager", () => {
             pullRequest: null,
             activeSessionId: null,
             sessionsSummary: [],
+            primaryUserLogin: null,
+            primaryUserAvatarUrl: null,
           },
         ],
       },