feat(sandbox-vms): wire Coder workspaces to docker-vm and talos-vm sandboxes

[ppat]

Summary

Automates the manual setup that made the KubeVirt sandbox VMs usable from a Coder workspace (docker-vm for DOCKER_HOST over SSH, talos-vm for a disposable Talos cluster), so it survives a workspace rebuild instead of being redone from memory each time.

• SSH: ~/.ssh/config + a dedicated known_hosts.sandbox pin docker-vm's now-stable host key (SHA256:uCqIXbsfvNMxdq8RPjy/CAiUFNFezWbNt+I8JtcGxt4, verified live) and point IdentityFile at a dedicated key with IdentitiesOnly yes — not ssh-agent, which is per-shell and doesn't survive a new terminal. UserKnownHostsFile points at a chezmoi-owned dedicated file rather than the main known_hosts, so managing it declaratively doesn't clobber entries ssh itself accumulates for other hosts.
• DOCKER_HOST is set in private_dot_env.tmpl once the sandbox key exists on disk (guards against pointing at a dead host before the secret's been fetched).
• talosctl: context/endpoint/node are fixed up on every apply (run_after_63_sandbox.sh.tmpl), since a talosconfig restored from Bitwarden can come back with an empty top-level context: field.
• kubeconfig merge: the sandbox kubeconfig is merged into ~/.kube/config via a temp-file flatten (KUBECONFIG=a:b kubectl config view --flatten > a truncates a before it's read — has to go through a temp file and mv), with the existing config listed first so current-context is never silently switched onto the sandbox.
• act: ~/.config/act/actrc maps ubuntu-24.04 (these repos' runs-on pin, which act's default map doesn't cover until ppat/github-workflows#577) and ships act's own "Medium" tier, so nobody hits the first-run image-size prompt or picks the insufficient Micro image by accident.
• Machine config decision: controlplane.yaml embeds the sandbox cluster's CA private key, and the workspace home PVC is shared RWX across workspaces. It's kept off disk by default — executable_fetch-sandbox-talos-machine-config fetches it via bws on demand instead of through chezmoi apply. The talosconfig alone (chezmoi-managed) covers all day-to-day use; the machine config is only needed for disaster recovery (talosctl gen secrets --from-controlplane-config).

Degrades gracefully

The only live network call this makes against the VMs is the talosctl kubeconfig fetch (talosctl's config context/endpoint/node are local-only file edits). That call is timeout-bounded and fails soft — logs a warning and leaves the existing kubeconfig untouched — so a down or nonexistent sandbox never blocks a workspace from starting. Verified by pointing the endpoint at a dead hostname and confirming graceful skip with a byte-identical kubeconfig before/after.

Known gap

sandbox_docker_vm_ssh_private_key does not exist yet in Bitwarden Secrets Manager — only its public half (sandbox_docker_vm_ssh_public_key) was ever uploaded. private_sandbox_docker_vm.tmpl carries a placeholder secret ID and a TODO with the exact bws secret create command to close the gap (this doesn't affect CI: these private_dot_ssh/*.tmpl/private_dot_talos/*.tmpl files aren't in any lint job's file set, same as the pre-existing private_id_ed25519.tmpl). Everything else in this change was exercised live against the real VMs from a workspace.

Test plan

• pre-commit run --all-files clean
• Rendered every new/changed template through the same sed+chezmoi execute-template pipeline CI uses, then shellchecked the output
• SSH: ssh docker-vm docker version and docker system dial-stdio both verified live over the managed config
• act -l and act -j <job> -n verified live against the remote daemon, actrc fix confirmed (ran through an entire job's steps before an artificial timeout cut it off)
• talosctl: config context/endpoint/node + talosctl get members verified live
• kubeconfig merge: verified live end-to-end (kubectl --context admin@sandbox-talos get nodes), current-context preservation verified with a simulated existing config
• Degradation path: endpoint pointed at a dead hostname, confirmed graceful skip + untouched kubeconfig, then restored
• CI

Closes #710