Repoint CI/CD: dev branch deploys to the staging VM

[crtahlin]

Part of #240. Depends on #SUB3, #SUB4.

Repoint CI/CD so the dev branch deploys to the staging VM instead of the current shared host.

Current state

.github/workflows/deploy.yml writes /opt/swarm_connect_dev.env and recreates the stack on the existing runner host for dev; main does the same with /opt/swarm_connect.env. This issue changes only the staging (dev) path; prod stays as-is until the prod epic.

Scope

• Deploy to the staging VM via SSH (ssh-action or a self-hosted runner on the VM). Add deploy secrets: host, user, SSH key.
• Move the staging env block (incl. the Flow B / Gnosis vars already added) to target the new VM; set GATEWAY_ENVIRONMENT=staging and the testnet chain values.
• Retire the old shared-host dev deploy path.

Acceptance criteria

• A push to dev deploys to the staging VM end-to-end.
• Old shared-host dev deploy removed/disabled.
• Deploy credentials stored as GitHub secrets (never vars); GNOSIS_PRIVATE_KEY remains a secret.
• Documented in the runbook (#SUB6).

[crtahlin]

Done. deploy.yml deploys the dev branch to provenance_gateway_dev on the new host.

It recreates only the service the deploying branch owns (#288). Before that, a push to either branch recreated every service, so deploying staging restarted production — and each restart made the stamp pool buy a reserve, roughly 0.7 BZZ for nothing.