chore: enable web commit sign-off requirement

[jflowers]

Context

The governance scan (2026-08-21) flagged web_commit_signoff_required: False as a gap.

Action

Enable Require contributors to sign off on web-based commits in:
https://github.com/organizations/unbound-force/settings/member_privileges

This is an org admin UI toggle, not manageable via peribolos or safe-settings.

Acceptance Criteria

• web_commit_signoff_required is True in the next governance scan