Allow more expensive models in subagents (with user approval)

[kycutler]

Testing #309322

Not exceeding the current cost tier is a good precaution -- but in some cases it's useful; e.g. running a subagent with a large context window to scan a large number of files and report a summary.

It might be nice if, when requesting a subagent with a more expensive model, the user can choose to approve or deny (or default to deny in e.g. autopilot mode)

[CodeByPeete]

Better suggestion

Make the main request cost the highest premium request factor in the tree of subagents.

Examples

1. GPT-5.4 Agent (1x) -> Opus 4.6 Subagent (3x) = 3x premium requests
2. GPT-5.4-mini Agent (0.33x) -> Opus 4.6 Subagent (3x) -> GPT-5.4 Subagent (1x) = 3x premium requests

This can be automatic imo then, too, since the user has to explicitly configure this, so the higher cost can be expected.