fix(cluster): preserve gateway image transport mode

[pimlock]

Summary

Keep tasks/scripts/cluster-deploy-fast.sh aligned with the active gateway image transport mode instead of defaulting to 127.0.0.1:5000/openshell.

Related Issue

N/A

Changes

• derive the gateway image repo base from the active cluster configuration before falling back to defaults
• distinguish direct-import mode from registry-backed mode during fast deploy
• import openshell/gateway:dev directly into k3s containerd when the cluster is using local image import mode
• only use the local registry path when the running gateway has a matching k3s mirror configured
• fail early with a clear message when a local registry repo is requested without corresponding k3s registry wiring

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Notes:

• mise run cluster succeeded locally after the fix, including the fast-deploy path.
• mise run pre-commit does not pass cleanly in this environment because python:proto is missing grpc_tools, one integration test expects port 8080 to be free, and repo-wide Clippy warnings are currently failing the task.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)