Fix: Reduce GitHub API rate limit usage by 75%

[bborn]

Summary

• Reduced GitHub API polling from every 15 seconds to every 60 seconds
• Increased PR info cache TTL from 15 seconds to 90 seconds
• Added rate limit error detection and graceful handling

Problem

The UI was hitting GitHub rate limits due to aggressive polling:

• Polled GitHub every 15 seconds
• Made 3 API calls per repository per poll (open, merged, closed PRs)
• 720 API calls per hour for a single repo
• Exceeded GitHub's 60/hour unauthenticated limit in just 5 minutes
• Used 14% of the 5,000/hour authenticated limit

Solution

API Usage Impact:

• Before: 720 API calls/hour per repo
• After: 180 API calls/hour per repo
• 75% reduction in API usage

Changes:

1. Refresh interval: 15s → 60s (4x reduction in frequency)
2. Cache TTL: 15s → 90s (ensures cache hits between refreshes)
3. Error handling: Detects rate limit errors and returns gracefully

Test Plan

• Existing tests pass
• Code compiles successfully
• Manual testing: Run UI for extended period to verify rate limits not hit
• Verify PR status updates still feel responsive (60s is acceptable for CI checks)

Files Changed

• internal/github/pr.go: Increased cache TTL, added rate limit detection
• internal/ui/app.go: Increased refresh tick interval from 15s to 60s

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