resolveAgentModelPrimary() only checks the agent-level model config and
does not fall back to the system-wide default. When users configure a
non-Anthropic provider (e.g. Gemini, Minimax) as their global default
without setting it at the agent level, the slug-generator falls through
to DEFAULT_PROVIDER (anthropic) and fails with a missing API key error.
Switch to resolveAgentEffectiveModelPrimary() which correctly respects
the full model resolution chain including global defaults.
Fixes#25365