[FEATURE] Per-call `effort` parameter on the Agent tool

The Agent tool takes model per call but not effort. Right now a subagent's effort comes from its agent definition (effort in frontmatter or --agents JSON) or falls back to the session level. The only place the orchestrating model can pick effort per spawn is Workflow agent().

So the main session can choose which model runs a task but not how hard it thinks, unless it goes through a Workflow or I define one agent type per effort level (reviewer-low, reviewer-high, ...), which clutters the agent list.

My use case: I run the main session at medium and want it to decide per spawn, e.g. low for a mechanical batch, xhigh for a tricky review, same agent type.

Proposal: an optional effort param on the Agent tool (low | medium | high | xhigh | max), same semantics as Workflow agent({ effort }). Precedence would be call > agent definition > session.

Related: #43083 and #67647 added definition-level and Workflow-level effort, this is the per-call piece.