Silence stale platform source 404s

[laclance]

Summary

Treat missing GitHub issues discovered from third-party bounty-platform feeds as the stale references they already semantically are, without printing them as scanner API failures.

Private run #13 surfaced two examples:

• sentryco/Telemetry#2
• buape/kiai-bounties#1

Both repositories are now absent on GitHub. issue_from_github_url_checked already returns (None, None) for HTTP 404 so these candidates are skipped without a source failure, but it logged GitHub API Error (not found) before applying that rule.

This moves the log after the 404 stale-reference check.

Regression coverage

• HTTP 404 remains a silent stale-reference skip
• non-404 API failures still emit GitHub API Error
• non-404 failures still return SourceFailureReason

Scope

Only:

• opportunity_scout/github.py
• tests/test_github.py

No platform parsing, candidate policy, private-instance, or release changes.