Add caching for improved performance

[steveclarke]

Description

Cache frequently accessed data to reduce API calls and improve CLI responsiveness.

What to cache

• Workspace list (for name resolution)
• Space list per workspace
• Board list per space
• User list per workspace (for name resolution)
• Tag list per workspace

Cache strategy

• File-based cache in ~/.cache/superthread/ or XDG cache dir
• TTL-based expiration (e.g., 5 minutes for lists)
• Manual cache clear: suth cache clear
• Cache status: suth cache status
• Disable with --no-cache flag or config option

Benefits

• Faster name-to-ID resolution
• Reduced API rate limit usage
• Better offline-ish experience for read operations

[steveclarke]

Additional: Cache current user info

When running suth setup or suth users me, we should cache the current user's info:

• user_id - needed for resolving "me" in commands like suth cards assigned me
• display_name - for UI display ("Logged in as: Steve")

This avoids an extra API call every time "me" needs to be resolved to a user ID.

Could be stored in cache or state file since it's tied to the account.