When an agent config specifies `model: { primary: "..." }` without
an explicit `fallbacks` array, the existing code replaced the entire
model object from `agents.defaults`—discarding the default fallbacks.
This caused cron jobs (and agent sessions) to have only one model
candidate (the pinned model) plus the global primary as a final
fallback, skipping all intermediate fallback models.
The fix merges the agent model override into the existing defaults
model object using spread, so that keys like `fallbacks` survive
when the agent only overrides `primary`. Agents can still explicitly
override or clear fallbacks by providing their own `fallbacks` array.
Reproduction scenario:
- `agents.defaults.model = { primary: "codex", fallbacks: ["opus", "flash", "deepseek"] }`
- Agent config: `model: { primary: "codex" }`
- Cron job pins: `model: "flash"`
- Before fix: fallback candidates = [flash, codex] (3 models lost)
- After fix: fallback candidates = [flash, opus, deepseek, ..., codex]
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