Support publish-mode app model introspection with aspire describe

## Description

`aspire describe` should be able to describe the app model in **publish mode**, not just describe a currently running AppHost.

Today, `aspire describe` is primarily useful for introspecting a running AppHost instance. For deployment and publishing workflows, we also need a way to inspect the publish-time app model without starting the app. The output should probably be different from the running-AppHost output because the questions are different.

In publish mode, the command should help answer things like:

- What resources are in the publish/deployment model?
- What deployment artifacts or steps would be produced?
- What resources participate in deployment, and how are they connected?
- What information does deployment need in order to list steps and reason about resources?

This may be related to, or potentially replace some use cases for, the manifest: a structured way to introspect the app model for publishing/deployment without relying on a running AppHost.

## Expected behavior

There should be a supported way to run something like `aspire describe` in publish mode and get structured output for the publish/deployment model.

The output should be distinct from the running-AppHost description and focused on deployment introspection: resources, publish artifacts, steps, and relationships needed by deployment tooling.

## Why this matters

Deployment tooling needs to inspect the app model, list deployment steps, and understand the resources involved without requiring the AppHost to be running. A publish-mode describe command would provide a clear CLI/API surface for that introspection.

Start at the existing `aspire describe` CLI entry point and trace how it produces the running-AppHost description. Compare that flow with the manifest and publish/deployment model surfaces mentioned in the issue; done means a supported publish-mode command returns structured resources, artifacts or steps, and deployment relationships without starting the AppHost.

Written by the indexing model from the issue text.