Agents should auto-install FlowFuse and certified nodes instead of asking the user

## Current behaviour

When an agent, whether the first party Expert or a third party agent over MCP, builds a flow that needs a node that isn't installed, it opens the editor's palette manager with the package pre-selected and the user confirms the install. This is by design, a deliberate security decision: agents don't get to put arbitrary code into a runtime, the user always has the final say.

The same applies at instance creation. `platform_create_hosted_instance` takes no module list, so even when the agent already knows the flow it is about to build needs certain nodes, it can't provision them up front.

## What we want

FlowFuse scoped plugins and certified nodes are vetted by us, so for those specific packages the security concern doesn't apply and we can skip the confirmation. Concretely, while a first or third party agent is driving the automation:

1. When the agent creates an instance and knows it will need certain nodes later, those nodes should be preinstalled as part of creation. This also sidesteps the fact that installing into a running instance can require a restart depending on the package.
2. When the agent needs one of these nodes on an existing instance while building a flow, it should install it automatically instead of sending the user to the palette manager.

Anything outside those two scopes keeps the current confirmation behaviour, and the manual (non-agent) flow doesn't change.