release: widen verifier timeout headroom

[resoltico]

Summary

• raise the default PR Gate and merge-handoff verifier timeout headroom
• document the explicit timeout override on both sides of the release protocol
• lock the new timeout contract into the release-surface regression tests

Verification

• ./scripts/test-verify-release-pr-gate.sh
• ./scripts/test-verify-release-merge-handoff.sh
• ./check.sh --console=plain
• git diff --check

[resoltico]

Superseded by the 0.42.0 release line and the merged verifier headroom changes that shipped with it.