Add 'validate' (Go vet/compile-check) as a required status check in branch ruleset

[jordigilh]

Context

While auditing #39, found that this repo's branch ruleset for main requires
DCO and ShellCheck as status checks, but not validate (the
validate-tests.yaml job that runs go vet --tags=e2e and a compile-check
over tests/e2e/).

Verified via:

gh api repos/dcm-project/utilities/rulesets/<id>

required_status_checks.required_status_checks = [{"context":"DCO"},{"context":"ShellCheck"}] only.

Why this matters

Any PR that only touches Go test files (like #39) can, in principle, merge
even if validate fails or is skipped, since it's advisory rather than
blocking. It's the one signal that would catch a Go compile break in the
shared E2E harness before it lands on main and breaks every consumer's
e2e run.

Ask

Add validate to the ruleset's required_status_checks list alongside
DCO/ShellCheck.