feat(issue): Add `explain` and `plan` commands (Seer AI)

[betegon]

Summary

Add AI-powered issue analysis commands using Sentry's Seer API.

Commands

sentry issue explain <issue-id>   # Get AI explanation of the issue
sentry issue plan <issue-id>       # Get AI-suggested plan for fixing the issue

Implementation

• PR feat: Add explain and plan commands (Seer AI)  #39 implements this feature

Acceptance Criteria

• sentry issue explain shows AI-generated explanation of the issue
• sentry issue plan shows AI-generated fix suggestions
• Commands work with short IDs and aliases
• Proper error handling for issues without AI data

Closes when PR #39 is merged.