fix(dv-pod): mount ENR private key at non-nested path in DKG sidecar

[nickh-obol]

Summary

The dkg-sidecar init container mounts the `charon-enr-private-key` secret as a subPath file at `/charon-data/charon-enr-private-key`, nested inside the `/charon-data` PVC mount. On k3d / Docker Desktop on macOS this triggers a runc `outside of rootfs` error at pod creation, blocking deployment entirely.

The same secret is already mounted non-nested at `/enr-from-job`, so the nested mount is redundant. This PR removes it and points `CHARON_PRIVATE_KEY_FILE` at the existing path. The dkg-sidecar honors the env var (source), so no sidecar code change is required.

Changes

• `charts/dv-pod/templates/statefulset.yaml`: drop the nested `/charon-data/charon-enr-private-key` volumeMount on the dkg-sidecar; repoint `CHARON_PRIVATE_KEY_FILE` to `/enr-from-job/{{ include "dv-pod.enrSecretDataKey" . }}`.
• `charts/dv-pod/Chart.yaml`: bump chart version 0.18.1 → 0.18.2.
• `charts/dv-pod/README.md`: regenerated by helm-docs.

The cluster-lock configMap mount under `/charon-data` was left as-is; it can be addressed separately if needed.

Test plan

Static

• `helm template` diff vs main: only the two intended substantive lines change (env var + dropped mount), rest is chart-version label bump.
• `helm lint` passes.

Live hoodi cluster (linux, local-path SC)

Upgraded all 4 `my-dv-pod-{0..3}` releases in the `dv-pod` namespace to `0.18.2` and brought the two scaled-down replicas back up.

• All 4 pods reach `2/2 Running` within ~75s.
• Pod events: no `FailedMount`, no `outside of rootfs`.
• `CHARON_PRIVATE_KEY_FILE` resolves to `/enr-from-job/charon-enr-private-key` on all 4 pods.
• dkg-sidecar volumeMounts list contains only `/enr-from-job` and `/charon-data` (no nested entry).
• ENR private-key sha256 unchanged across upgrade for all 4 secrets.
• dkg-sidecar logs: `Cluster lock file already exists. DKG is complete.` (early-exit path).
• Charon JSON logs: 0 errors across all 4 pods. Only benign warnings.
• All 4 nodes connected as peers via relay (`p2p_peer_connection_total` = 1 between every pair).

macOS / k3d

• Reproduced the original `outside of rootfs` error against current released chart.
• Same values against this branch: pod reaches `Running`, dkg-sidecar starts cleanly.

Related

PR #163 addresses a similar but distinct issue (`cluster-definition.json` write under `/charon-data` causing `EACCES` on `local-path`). Both stem from piling files under the `/charon-data` PVC. Scoping this fix narrowly to the ENR mount; #163 can be picked up separately.