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
The slug generator was using hardcoded DEFAULT_PROVIDER and DEFAULT_MODEL
instead of resolving from agent config. This caused it to fall back to
anthropic/claude-opus-4-6 even when a cloud model was configured.
Now uses resolveAgentModelPrimary() to get the configured model, with
fallback to defaults if not configured.
Fixes issue where session memory filenames would fail to generate
when using cloud models that require special backends.
