feat: add follow/unfollow commands

- bird follow <username-or-id>: follow a user by handle or ID
- bird unfollow <username-or-id>: unfollow a user by handle or ID

Uses REST API /1.1/friendships/create.json and /destroy.json with GraphQL fallback.
Accepts @handles, bare usernames, or numeric user IDs.

src/lib/twitter-client-types.ts

@@ -228,6 +228,17 @@ export type BookmarkMutationResult =
       error: string;
     };
 
+export type FollowMutationResult =
+  | {
+      success: true;
+      userId?: string;
+      username?: string;
+    }
+  | {
+      success: false;
+      error: string;
+    };
+
 export interface UploadMediaResult {
   success: boolean;
   mediaId?: string;