fix: trim cron model overrides and doc guidance (#711) (thanks @mjrussell)

docs/automation/cron-jobs.md

@@ -69,6 +69,10 @@ Isolated jobs (`agentTurn`) can override the model and thinking level:
 - `model`: Provider/model string (e.g., `anthropic/claude-sonnet-4-20250514`) or alias (e.g., `opus`)
 - `thinking`: Thinking level (`off`, `minimal`, `low`, `medium`, `high`)
 
+Note: You can set `model` on main-session jobs too, but it changes the shared main
+session model. We recommend model overrides only for isolated jobs to avoid
+unexpected context shifts.
+
 Resolution priority:
 1. Job payload override (highest)
 2. Hook-specific defaults (e.g., `hooks.gmail.model`)