When replace_plan is true (default), the native plan agent was demoted to
subagent but remained visible in the agent picker. This caused Sisyphus to
route planning to the native plan agent instead of Prometheus.
Add hidden: true to buildPlanDemoteConfig(), consistent with how the build
agent is hidden when default_builder_enabled is false.
Previously, demoted plan agent only received { mode: 'subagent' } with no
model settings, causing fallback to step-3.5-flash. Now inherits all
model-related settings (model, variant, temperature, top_p, maxTokens,
thinking, reasoningEffort, textVerbosity, providerOptions) from the
resolved prometheus config. User overrides via agents.plan.* take priority.
Prompt, permission, description, and color are intentionally NOT inherited.
YeonGyu-Kim