ci: lint workflows and shell scripts, validate k8s against the API server

Adds three lint jobs (actionlint, shellcheck, compose) and a server-side
dry-run of the active manifests. Previously the only k8s check was
kubeconform, which has no schemas for CRDs, so every IngressRoute,
Certificate, PrometheusRule and Middleware was silently skipped.

The server-side pass needs the live API server because that is the only
place the real CRD schemas and the cert-manager / Traefik admission
webhooks exist. It is scoped to services carrying a k8s/active marker,
since dry-run needs the target namespace to exist. userbot/ is excluded
from shellcheck: it is a git subtree, and linting upstream's scripts would
let a routine subtree pull turn the deploy gate red on code we do not own.

kubeconform, shellcheck and actionlint are now installed from pinned
versions in tool-versions.env rather than picked up from the runner's
PATH. The Compose helper is shared with the deploy workflow so both
check the same file set the same way.

.gitea/workflows/tool-versions.env

@@ -0,0 +1,9 @@
+# Pinned versions of the CI linters installed by install-ci-tools.sh.
+# Renovate keeps these up to date (see customManagers in renovate/renovate.json).
+#
+# The renovate image version is NOT pinned here: renovate/k8s/cronjob.yaml is the
+# single source of truth and the workflows read the tag from it, so there is
+# nothing to drift.
+ACTIONLINT_VERSION="1.7.7"
+SHELLCHECK_VERSION="0.11.0"
+KUBECONFORM_VERSION="0.8.0"