fix(issue-discovery): ignore zero-share repos

[Desel72]

Summary

Filters issue-discovery inputs to repositories whose issue_discovery_share is greater than zero, so PR-only repos cannot unlock issue-discovery eligibility or produce raw discovery rows.

Related Issues

Fixes #1267

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other (describe below)

Testing

• Tests added/updated
• Manually tested

uv run pytest tests/validator/issue_discovery/test_scan.py -q
uv run ruff check gittensor/validator/issue_discovery/scan.py tests/validator/issue_discovery/test_scan.py
uv run ruff format --check gittensor/validator/issue_discovery/scan.py tests/validator/issue_discovery/test_scan.py

Checklist

• Code follows project style guidelines
• Self-review completed
• Changes are documented (if applicable)