feat: implement GitHub API failure recovery and jittered backoff

[saidai-bhuvanesh]

Fixes #6105

Phase 3: GitHub API Failure Recovery

• Added jittered exponential backoff retries (getJitteredBackoff) to handle transient rate limits or outages without overloading GitHub.
• Added intelligent error classification (shouldFallbackOnError) to distinguish transient errors from fatal logic/auth exceptions.
• Implemented stale-cache and placeholder mock-data fallbacks in fetchUserProfile, fetchUserRepos, and fetchGitHubContributions.
• Added the isOfflineFallback flag to return data so the UI can notify the user of potentially outdated information.
• Wrote integration tests in tests/github-recovery.test.ts.

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Unfortunately, this PR has been automatically closed because you are not assigned to the linked issue #6105 — feat(api): intelligent GitHub API failure recovery and mock fallbacks.

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