Add user ID resolution (-u, -f flags)

[wham]

Replace raw user IDs with Slack handles in exported JSON.

• -u / --users: resolve user IDs to Slack handles (message authors, reactions, mentions, thread replies)
• -f / --force: force re-fetch of the cached user list (implies -u)
• User list is fetched per-page with rate limit retry and cached per-workspace in the gh CLI cache directory
• Cache stores only IDs and handles