feat: make GitHub searches resilient

[techgaun]

Summary

• construct GitHub clients lazily and allow client injection for testing
• replace iterator copying with bounded retries on the active search
• respect Retry-After and search rate-limit reset metadata
• use exponential backoff for GitHub server errors and connection timeouts
• add --max-retries with a default of three retries per query
• continue scanning after an individual query fails
• keep authentication failures fatal
• report query, match, failure, retry, and elapsed-time totals
• return exit code 0 for complete scans, 2 for partial scans, and 1 for fatal errors
• propagate exit codes through both Python entry points

Compatibility

Existing repository/user, dictionary, monitoring, and CSV flags remain supported. Default retry behavior is bounded and can be disabled with --max-retries 0.

Verification

• python3 -m unittest discover -s tests -v (18 tests)
• flake8 in Python 3.12
• git diff --check
• Docker image/package build
• Docker CLI version smoke test
• Docker missing-dictionary smoke test returned exit code 1