[Bug] scan_closed_issues round-trips lookback datetime → ISO string → datetime with hand-rolled 'Z' replace

[atoz96]

Description

scan_closed_issues in gittensor/validator/issue_discovery/repo_scan.py:76 builds lookback_date as an ISO string, then _scan_repo:134 parses it back to a datetime via datetime.fromisoformat(lookback_date.replace('Z', '+00:00')). The string is only used by _fetch_closed_issues (REST since param), but the same datetime is required for the per-issue closed_at cutoff. Round-tripping introduces a class of bugs (wrong timezone parse, format drift if the strftime template ever changes).

Steps to Reproduce

1. Inspect gittensor/validator/issue_discovery/repo_scan.py:76, 134.
2. Note the strftime('%Y-%m-%dT%H:%M:%SZ') followed by fromisoformat(... .replace('Z', '+00:00')).

Expected Behavior

Compute lookback_dt once as a datetime, pass that into _scan_repo, and stringify only at the API boundary in _fetch_closed_issues.

Actual Behavior

A two-step datetime → string → datetime roundtrip with hand-rolled 'Z' handling.