feat: add issue subscribe/unsubscribe commands

[eslerm]

Add issue subscribe and issue unsubscribe commands to follow/unfollow issue notifications.

Scope

• 2 new mutations: issueSubscribe, issueUnsubscribe
• Extend existing issue command group, client, and GraphQL mutations
• Issue ID resolution already works (ENG-123 or UUID)
• Estimated ~165 LOC

Open Questions

• Should subscribe accept --user to subscribe someone else, or just current user?

See planning/07-issue-subscriptions.md