repo: protect main and require the validate check

[TMHSDigital]

gh api repos/TMHSDigital/Free-Game-Dev-Assets/branches/main/protection returns 404 and there are no rulesets. Every catalog rule lives in validate.mjs, but nothing stops a push that fails it.

Suggested:

• A ruleset on main requiring the validate status check (for PRs, at least; allow the owner to bypass if direct pushes are the workflow).
• Enable "Automatically delete head branches" (deleteBranchOnMerge is currently false).

[TMHSDigital]

Deferred by the maintainer on 2026-09-26: direct pushes to main stay the workflow for now. The validate check now runs on every push and every PR (#1, merged in #49), so a failing push shows up immediately even without a ruleset.