[Bug]: No cloud-init Extra map key collision validation

[mrhillsman]

Bug Description

`internal/cloudinit/cloudinit.go` (lines 69-72) merges arbitrary keys from `opts.Extra` into the cloud-config document without checking for collisions with standard keys (`packages`, `runcmd`, `write_files`, `users`, `ssh_pwauth`). A workload that includes any of these standard keys in its Extra map will silently overwrite the builder's values, producing a broken cloud-init config.

Steps to Reproduce

1. Create a workload that sets `opts.Extra["packages"] = [...]]` with custom packages
2. The cloud-init builder also sets `packages` from its own logic
3. The Extra merge overwrites the builder's package list
4. The resulting cloud-init config is missing required packages

Expected Behavior

Either validate that Extra keys don't collide with reserved keys (returning an error), or merge list/map-type values instead of overwriting.

Actual Behavior

Extra keys silently overwrite standard cloud-config keys, producing broken configurations.

Command

virtwork run

Go Version

Not provided

Deployment Mode

Not provided

Additional Context

Severity: Medium

Files:

• `internal/cloudinit/cloudinit.go:69-72` — unchecked `Extra` merge