feat(github): add retry-with-backoff logic for repository update failures

[victorisiguzoruzoma874]

Summary

• Add exponential backoff retry with jitter for transient GitHub API failures
• Distinguish retryable (5xx, 429, network) from non-retryable (4xx) errors
• Integrate into GitHub repository update service for graceful failure handling
• Fail fast on auth errors, retry only transient failures

Implementation Details

• Retry configuration: 5 attempts, 200ms initial, 30s max delay, 5min total duration
• Jitter (±10%) prevents thundering herd in concurrent scenarios
• Error detection: HTTP status codes, network/timeout patterns
• Full test coverage for retry behavior across error types

Key Design Decisions

• Skip retry on 4xx (except 429) to fail fast on validation/auth errors
• Jitter on backoff delays to prevent synchronized retries
• Cap total duration to prevent indefinite retry loops

closes #608

🤖 Generated with Claude Code

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