[codex] Store etcd snapshots in Longhorn PVC

[boxp]

Summary

• Store Kubernetes upgrade etcd snapshots in a Longhorn PVC exposed by the GitOps-managed etcd-snapshot-store Deployment.
• Remove the GitHub Actions runner fetch/S3 upload path that was failing on large snapshot transfer.
• Keep the host-side snapshot for the existing rollback task and prune old PVC snapshots by retention count.
• Improve Plan Ansible CI failure diagnostics by printing captured stderr and JSON tail, and uploading plan outputs on failure.
• Add the implementation plan under docs/project_docs/BOXP-2-etcd-snapshot-longhorn-pvc/plan.md.

Validation

• git diff --check
• ansible-playbook --syntax-check -i inventories/production/hosts.yml playbooks/upgrade-k8s.yml in Docker via ghcr.io/astral-sh/uv:python3.14-bookworm
• ansible-lint playbooks/upgrade-k8s.yml roles/kubernetes_upgrade/tasks/etcd_snapshot.yml in Docker via ghcr.io/astral-sh/uv:python3.14-bookworm
• molecule test for roles/kubernetes_upgrade in Docker with Docker CLI installed in the validation container
• codex review --uncommitted

Notes

The old S3 Terraform resources are intentionally left managed for now so Terraform does not try to delete a potentially non-empty bucket during this migration.

This PR expects the GitOps-managed storage app from boxp/lolice#587 to be applied before running the Kubernetes upgrade pre-check.

[github-actions]

Ansible Plan Results

Mode: --check --diff (dry run)

⚠️ Changes detected

---

shanghai-1: control-plane

Host	OK	Changed	Skipped	Failed	Unreachable
shanghai-1	73	1	12	0	0

1 changed

Changed Tasks (1)

#	Task	Module
1	user_management : Update package cache	unknown

shanghai-1: node-shanghai-1

Host	OK	Changed	Skipped	Failed	Unreachable
shanghai-1	9	0	0	0	0

No changes

shanghai-2: control-plane

Host	OK	Changed	Skipped	Failed	Unreachable
shanghai-2	73	1	12	0	0

1 changed

Changed Tasks (1)

#	Task	Module
1	user_management : Update package cache	unknown

shanghai-2: node-shanghai-2

Host	OK	Changed	Skipped	Failed	Unreachable
shanghai-2	9	0	0	0	0

No changes

shanghai-3: control-plane

Host	OK	Changed	Skipped	Failed	Unreachable
shanghai-3	73	1	12	0	0

1 changed

Changed Tasks (1)

#	Task	Module
1	user_management : Update package cache	unknown

shanghai-3: node-shanghai-3

Host	OK	Changed	Skipped	Failed	Unreachable
shanghai-3	9	0	0	0	0

No changes

---

Plan executed on all nodes in parallel.