Add feature flag authorization review gates

[Errordog2]

/claim #2225

Summary

• Adds a feature-flag authorization boundary gate to the secure-code-review skill.
• Distinguishes rollout, experiment, kill-switch, authorization-policy, and temporary privileged flags.
• Requires trusted flag source, independent server-side authorization, fail-closed behavior, owner/expiry/cleanup evidence, and tests proving hidden UI does not imply API denial.
• Updates the output template, review checklist, common pitfalls, and changelog for secure-code-review v1.0.1.

Why

Issue #2225 notes that feature flags can be over-credited as authorization and that client-controlled or stale privileged flags can leave sensitive API paths reachable. This change makes reviewers prove the authorization boundary separately from rollout or experiment logic.

Validation

• git diff --check
• Frontmatter required-field check across skills and roles
• index.yaml file existence check
• Prompt-injection phrase scan across skills and roles
• Targeted rg check for version and new feature-flag sections