chore(renovate): move config under renovate/ and validate it in CI

The config lived in renovate.json at the repo root while everything else
Renovate-related sat under renovate/, and renovate/config.js was a second,
unused source of truth. Both are gone: renovate/renovate.json is now the
only config file.

Because the CronJob in the cluster cannot read the repository, its
ConfigMap carries an inlined copy of the config. That copy is generated,
and sync-renovate-configmap.sh --check now fails the build when it drifts
from the source file.

The workflows also stop carrying a copy of the renovate/renovate image
tag. They read it from renovate/k8s/cronjob.yaml, so the version validated
in CI is the version that actually runs in the cluster.

ci.yaml validates the config with renovate-config-validator, checks the
generated ConfigMap, and kubeconforms the CronJob's own manifests.

renovate/README.md

@@ -26,15 +26,17 @@ from Git and must be applied separately after every new cluster.
 
 Run it immediately instead of waiting for the six-hour schedule.
 
-Two options, both use the same `renovate/config.js`:
+Two options, both use the same `renovate/renovate.json`:
 
 ```sh
 kubectl create job --from=cronjob/renovate renovate-manual-$(date +%s) -n renovate
 ```
 
 or the `renovate-run` Actions workflow (Actions tab → `renovate-run` →
-Run workflow). It runs `renovate/renovate:44.103.0` on the self-hosted
-runner via Docker. Required Actions secrets (repo or org settings):
+Run workflow). It runs the same image as the CronJob on the self-hosted runner
+via Docker — the tag is read out of `renovate/k8s/cronjob.yaml` at run time
+rather than hardcoded, so the two cannot drift apart. Required Actions secrets
+(repo or org settings):
 
 - `RENOVATE_TOKEN` — renovate-bot PAT (repository + issue read/write).
 - `RENOVATE_GITHUB_COM_TOKEN` — optional, for changelogs and GitHub rate limits.
@@ -64,6 +66,33 @@ docker compose -f renovate-compose.yaml run --rm renovate
 The Compose file is intentionally named `renovate-compose.yaml`, so the
 repository's automatic deployment discovery does not start it accidentally.
 
+## Configuration
+
+`renovate/renovate.json` is the single source of truth. The Compose file and the
+`renovate-run` workflow mount that file directly.
+
+A ConfigMap cannot read from the repository, so the CronJob needs the config
+inlined. `renovate/k8s/configmap.yaml` is therefore a **generated** copy:
+
+```sh
+.gitea/workflows/sync-renovate-configmap.sh          # regenerate after editing
+.gitea/workflows/sync-renovate-configmap.sh --check  # fail if out of date
+```
+
+The `renovate-ci` workflow runs the `--check` form on every PR and push, so a
+config edit that forgets to regenerate the ConfigMap cannot be merged.
+
+Beyond images, `customManagers` in the config track:
+
+- Helm chart versions pinned in `.gitea/workflows/deploy-lib.sh`. The built-in
+  `helmv3` manager only reads `Chart.yaml` and `helm-values` only reads values
+  files, so neither sees a version written into a `helm upgrade` command —
+  these are declared as `custom.regex` managers against the `helm` datasource.
+- CI linter versions in `.gitea/workflows/tool-versions.env`.
+
+The Renovate image tag is deliberately _not_ in `tool-versions.env`:
+`renovate/k8s/cronjob.yaml` owns it, and the workflows read it from there.
+
 ## How updates flow
 
 Renovate scans both `compose.yaml` files and Kubernetes manifests, opens a