Improve RBAC activation context evidence gates

[MAUROCERON]

Summary

• Implements [REVIEW] rbac-design: add temporary elevation and activation-context evidence gates #1327.
• Adds temporary elevation and activation-context evidence gates to rbac-design.
• Requires reviewers to distinguish assignment eligibility, activation event,
  active-session DSoD, token/session lifetime, and break-glass post-use review.
• Adds edge-case fixtures for short-lived approved activation, long-lived JIT
  without approval, DSoD checked only at assignment time, cached tokens outliving
  deactivation, and break-glass without post-use review.

Validation

• Checked Markdown fence balance for the edited skill and new fixture.
• Verified official NIST, Microsoft Entra PIM, and AWS IAM Identity Center reference URLs return HTTP 200.
• Scanned the changed public files for private payment/contact strings.

Bounty

• I have read and agree to the CONTRIBUTING.md bounty terms.
• Preferred payment method can be provided privately after maintainer acceptance.