fix(backstage): drop the portal's dead agentPlatform.modelManager block

[teemow]

What does this PR do?

Stops rendering the portal's dead model-manager entry into the Backstage app-config. Closes #318.

• templates/backstage/app-config.yaml: the modelManager.installations.<installation>.apiBaseUrl block under agentPlatform, and the route term that gated the agentPlatform wrapper on it, are gone. The Models pages call model-manager's x_model-manager_* tools through muster as the signed-in person (Models pages call model-manager through muster MCP as the signed-in user, not through the agent-platform-backend REST proxy backstage#2294, Backstage 2.19.0 and later; components.backstage.versionRange is >=1.0.0 <3.0.0), and the portal backend has no model-manager client that would read the key.
• make verify-wiring: with Backstage, model-manager and its route on, the app-config carries no model-manager entry and no apiBaseUrl ending in /model-manager; the muster.installations entry is still asserted.
• README (the model-manager route row, the Backstage wiring row), UPGRADE.md (operator action: none on Backstage ≥ 2.19.0; a Backstage peer in modelManager.networkPolicy.ingress.additionalPeers can go), CHANGELOG.

The model-manager route itself (modelManager.route) and model-manager's MCPServer (its own chart) are unchanged. The fleet render with Backstage off is unchanged: the template renders only with components.backstage.enabled.

How does it look like?

make verify-wiring passes. The agentlab proof (the Models pages as a signed-in user over muster with this chart) is recorded before merge.

[circleci-architect]

Published Helm chart agent-platform-connectivity

4.69.2-r1d640438t20260924202955hfbf40b5

Chart	agent-platform-connectivity
Version	4.69.2-r1d640438t20260924202955hfbf40b5
OCI reference	oci://gsoci.azurecr.io/charts/giantswarm/agent-platform-connectivity:4.69.2-r1d640438t20260924202955hfbf40b5
Digest	sha256:e0999ded09c27b0f0a0ab2469ca0ea4be56776994f936c01c69cba2f92dce19f
Registry	public — gsoci.azurecr.io
Git catalog	giantswarm-test-catalog (index)

Pull this chart

helm pull oci://gsoci.azurecr.io/charts/giantswarm/agent-platform-connectivity --version 4.69.2-r1d640438t20260924202955hfbf40b5

_{Posted by architect-orb · build 13378 · commit fbf40b5 · updated in place on every push}

[circleci-architect]

Published Helm chart agent-platform

4.69.2-r1d640438t20260924202955hfbf40b5

Chart	agent-platform
Version	4.69.2-r1d640438t20260924202955hfbf40b5
OCI reference	oci://gsoci.azurecr.io/charts/giantswarm/agent-platform:4.69.2-r1d640438t20260924202955hfbf40b5
Digest	sha256:36b3d5f441c2ce1ae9cfb7c94b0779ff63cadbd10a96bfed266ed582f2138059
Registry	public — gsoci.azurecr.io
Git catalog	giantswarm-test-catalog (index)

Pull this chart

helm pull oci://gsoci.azurecr.io/charts/giantswarm/agent-platform --version 4.69.2-r1d640438t20260924202955hfbf40b5

_{Posted by architect-orb · build 13398 · commit fbf40b5 · updated in place on every push}