GitHub security workshop: Add security overview, governance, and rollout exercise

[softchris]

Goal

Close the workshop by connecting repository alerts to triage operations and organization-wide security rollout decisions.

Scope

Teach alert ownership, prioritization, campaign or backlog planning, coverage visibility, metrics, exceptions, and rollout sequencing. Use organization Security Overview when available, with repository-level sample data and screenshots as a complete fallback for personal-account learners.

Acceptance criteria

• Learners review code scanning, secret scanning, and dependency findings in a unified triage exercise.
• The exercise prioritizes findings by exploitability, severity, exposure, and remediation availability rather than raw count alone.
• Learners assign an owner, target date, and disposition to a small sample backlog.
• Organization-level coverage and Security Overview concepts are explained with current entitlement requirements.
• A repository-level fallback provides equivalent learning when organization access is unavailable.
• The exercise covers staged enablement, developer communication, bypass and dismissal governance, remediation SLAs, and measuring adoption.
• Learners produce a concise rollout plan for a fictional shelter organization.
• Final cleanup confirms training alerts, vulnerable branches, test values, and temporary rulesets are removed.
• The workshop README includes a completion checklist and next-step resources.

[scubaninja]

Backlog review priority: deferred. Tagging @scubaninja for visibility and follow-up.