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.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@yuta/bird",
-  "version": "0.7.0",
+  "version": "0.8.0",
   "description": "CLI tool for tweeting and replying via Twitter/X GraphQL API",
   "type": "module",
   "main": "dist/index.js",