fix(gator): surface PR-owned security regressions in reviews

[johntmyers]

Summary

Clarify that security regressions introduced or newly exposed by an unmerged PR belong in its normal, actionable PR review. Prevent security classification alone from creating a private-triage gate or suspending operator-authorized tests.

Related Issue

Closes #4337

Changes

• Distinguish PR-owned regressions from pre-existing vulnerabilities independent of the PR in the Gator gate skill, top-level prompt, and independent reviewer guidance.
• Explicitly treat new reachable paths, trusted sinks, and security contracts as PR-owned even when an underlying unsafe primitive existed before the change.
• Preserve normal evidence, severity, blocker, and test-authorization rules; private handling under SECURITY.md remains applicable to pre-existing independent vulnerabilities.
• Require explicit operator authorization for external disclosure or reporting.
• Update launcher guidance, add instruction-consistency regression checks, and bump the immutable payload to 12. Payload 11 is reserved by the separate large-diff fix in fix(gator): handle oversized PR diffs in review ledger #4309; this PR does not include that implementation.
• Leave SECURITY.md and existing sandbox state unchanged. No detailed security finding is published by this change.

Testing

• Checks appropriate to the affected code and behavior pass: all four Gator helper suites, Bash syntax, manifest YAML/version validation, and git diff --check.
• Unit tests added/updated: regression assertions cover consistent review routing and external-disclosure authorization across the gate, prompt, and reviewer.
• Tests validate instruction text and helper behavior, not deterministic model compliance. No live watcher was relaunched with payload 12.
• Full Rust/SDK/E2E suites were not run; no runtime implementation changed.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)