fix(config): a missing platform.valuesFiles entry is reported as 'no agentlab.yaml found', and blocks down

[teemow]

Problem

When platform.valuesFiles names a file that no longer exists, agentlab down (and presumably every command that loads the configuration) fails with

no agentlab.yaml found

although agentlab.yaml is right there. Observed on 2026-09-12 while taking a lab down whose valuesFiles pointed at a checkout that had been removed: the message sent the operator looking for the wrong problem; editing the valuesFiles entry out made down work.

Proposed solution

A missing values overlay is reported as what it is — platform.valuesFiles[0]: <path>: no such file — and down in particular should not need the overlays at all (destroying the cluster needs the cluster name, nothing from the platform values).

Acceptance criteria

• A configuration whose platform.valuesFiles names a missing file fails agentlab render/up/platform with an error that names the key and the path.
• agentlab down succeeds on such a configuration.
• A unit test for the loader's message.