Record the Harness pass-through principles

[SaladDay]

Adds the design rule behind #760–#767 to AGENTS.md as "Core assembles, Harnesses run natively":

• Choosing a Harness and a model behaves like running it locally. The only differences are the cloud Environment, isolation, credentials and unattended operation.
• Core passes configuration through. It does no provider qualification, injects nothing into model input and has no fallback; a field with no native home gets a typed error.
• One declaration file joins a Harness.
• Defaults come from the official API when it defines one; everything else uses the native default.
• An adapter overrides a native default only for unattended cloud operation. Each override has a reason and is listed in its adapter guide.
• Nothing branches on the model or provider.

The Harness protocol links the rule. The Harness capabilities page says that anything it does not list follows native behavior. The README (EN/zh/ja) says each Harness runs as it does locally. The zh mirrors are updated, and tests/translations.test.mjs passes.

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