Release v3.1.1

Added

• PR branch validation CI gate: .github/workflows/validate-pr-branch.yaml blocks PRs to main unless from release/* or hotfix/* branches
• /prepare-release skill: Automates release branch creation, version bump, changelog, and PR to main
• /prepare-release command: Command gateway for the prepare-release skill

Changed

• RELEASING.md: Added CI enforcement section and documented /prepare-release as recommended release flow
• CLAUDE.md: Updated Git Workflow section with branching requirement and release process reference