[S6] Implement GitHub API rate limit tracking and backoff

[krakenhavoc]

Description

Systematic rate limit handling across all GitHub API calls.

Implementation

• Wrapper around Octokit that reads rate limit headers on every response
• Log current remaining/limit/reset after each call
• Automatic backoff when remaining drops below threshold (100)
• Exponential backoff on 403 rate limit responses
• Per-installation tracking (each installation has its own rate limit)

Acceptance Criteria

• Rate limits tracked per installation
• Automatic backoff prevents 403s
• Backoff is logged with reset time
• Works for both REST and GraphQL endpoints