Admit Substrate 1.8.0: the range bump with its snapshot reset #918

Problem

components.substrate{,-crds}.versionRange is >=1.7.0 <1.8.0 with prereleases: false, so no Substrate 1.8.0 candidate rolls anywhere by itself. The 1.8 line carries what the platform needs next:

• the Google access-token credential provider a Vertex AgentTemplate on Substrate binds to (Vertex ModelConfigs with apiKeySecret are refused on the Substrate actor runtime kagent-upstream#178; on the line since giantswarm/substrate v1.8.0-rc.2), so docs(valkey): record why muster-valkey AOF is deliberately not enabled #178's acceptance on an installation waits for this range move;
• the PATH_MAX snapshot fix (fix(snapshot): carry upstream's PATH_MAX-safe snapshot tars substrate#260, upstream snapshot rootfs uppers as one tar per container agent-substrate/substrate#2110), which is breaking: snapshots written before it do not resume after it, so every golden and actor snapshot is reset when the line rolls;
• the chart's own egress-gateway ephemeral-storage bound (fix(chart): bound the egress gateway's ephemeral storage substrate#255), which retires the kustomize patch of fix: atenet-egress emptyDir limits and oauth2-proxy registry on 4.x #880.

No issue in this repository names the range move; it is the consumer step of the Substrate 1.8.0 promotion.

Proposed solution

Once Substrate 1.8.0 is stable: prove the release in the agentlab with an explicit pin and the snapshot-reset procedure first, then bump both ranges to >=1.8.0 <1.9.0 (BOM, README, goldens, verify-components), drop the ephemeral-storage patch below the new floor, and name the snapshot reset in UPGRADE.md.

Acceptance criteria

• Substrate 1.8.0 rolls through the range on a test installation after its snapshot reset; a Vertex AgentTemplate there is Compatible=True and a turn completes (docs(valkey): record why muster-valkey AOF is deliberately not enabled #178's acceptance).
• The kustomize ephemeral-storage patch no longer renders with the 1.8 range.
• UPGRADE.md names the snapshot reset.

This issue was written by an agent.