feat(issue): Magic `@` selectors (`@latest`, `@most_frequent`)

[betegon]

Summary

Add magic @ selectors for quick issue access without needing to specify IDs.

Usage

sentry issue get @latest          # Issue with most recent event (lastSeen)
sentry issue get @most_frequent   # Issue with highest event frequency
sentry issue fix @latest          # Fix the most recent issue
sentry issue explain @latest      # Explain the most recent issue

Selectors

Selector	Resolves to
@latest	Issue with most recent lastSeen timestamp
@most_frequent	Issue sorted by event frequency (highest)

Implementation Notes

• Selectors should work with issue get, issue explain, issue fix
• Use issue list API with appropriate sort parameter
• Cache the resolved issue for subsequent commands in same session

Acceptance Criteria

• @latest resolves to issue with most recent lastSeen
• @most_frequent resolves to issue with highest frequency
• Works with get, explain, fix commands
• Clear error message if no issues found