feat: batch GitHub Search API with OR semantics for labels and repos

[cdhagmann]

Summary

• Replaces per-project per-label Issues API calls with a single batched GitHub Search API request per 10 repos
• Search query uses OR semantics: is:issue state:open (label:"A" OR label:"B") repo:o/r1 repo:o/r2 …
• Reduces N×labels network round trips to ceil(N/10) calls for both scan and issues all
• Results are cached by the full query string; same-lockfile repeat scans still hit cache

Linked issue / ADR

Closes #52 (reduces API call count to avoid rate limiting).

Working agreement

• No new architectural boundaries crossed — adapter change stays inside GitHubAdapter
• Specs updated for all three affected files (github_adapter_spec, scan_spec, issues_spec)
• bin/rubocop clean
• bundle exec rspec — 246 examples, 0 failures

Test plan

• #issues_matching_labels unit tests: hash return format, empty-inputs short-circuit, batching (11 repos → 2 requests), caching (same call → 1 request), OR query structure
• scan_spec fully rewritten to mock issues_matching_labels returning a hash
• issues_spec fully rewritten; batch-level failure test replaces per-gem failure test

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