Experimental Desktop runtime: agent idle protection

Source: 00c8ad14fdcfcf195c6e78b3a21667a7ddac2251.
Native Linux amd64 and arm64 builds, Go tests/race checks and real sandbox
lifecycle checks passed in CI 38833072219.

Exposes the existing idle_policy through public app-sandbox creation and reads,
with negotiated app_sandbox_idle_policy support. Defaults and existing apps
are unchanged. Malformed create requests fail before provisioning. Retains the
previous experimental scoped provider delivery and renewal APIs.

Desktop can opt new catalog agents into always_on; ordinary apps retain idle
sleep. This does not keep the host/VM awake, renew model access, or prevent
emergency memory-pressure stops. No schema, permanent daemon or licensing added.

Both real Linux OpenClaw/Hermes candidate workflows passed idle-reaper comparison,
authenticated preview, reload and restart recovery using synthetic credentials.
This is not native Windows/macOS or real-model validation. Existing installations
are not silently migrated. Experimental prerelease, not a production upgrade.

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