fix: count ALL open issues for spam multiplier instead of lookback-bounded subset (#929)

[alpurkan17]

Summary

Mirror issue discovery currently uses the same lookback-bounded get_miner_issues(since=lookback_date) response for both scoring (correct) and counting open issues for the spam multiplier (incorrect). Open issues created before PR_LOOKBACK_DAYS are omitted from total_open_issues and never trigger the open-issue spam gate.

This fix adds a second get_miner_issues call without the lookback window, then counts OPEN issues across mirror-enabled repos from that full response. The scoring path still uses the lookback-bounded list unchanged.

If the unfiltered fetch fails, the lookback-bounded count is used as a fallback approximation (previous behavior).

Validation

• total_open is derived from get_miner_issues(github_id) (no since) — counts ALL open issues
• total_open falls back to lookback-bounded count if the unfiltered fetch raises MirrorRequestError
• Scoring still uses the lookback-bounded filtered list
• ruff check + ruff format pass

Closes #929

[anderdc]

Duplicate of #930 which is better scoped. Closing.