feat: support effort level in agent definition frontmatter

Summary

Add effort as a supported frontmatter field in .claude/agents/*.md agent definition files, similar to the existing model field.

Motivation

Currently the only way to control effort level is globally via settings.json → effortLevel. But different agents have very different complexity profiles — a deal-hunting agent doing web scraping needs low effort, while a development agent implementing GitHub issues needs xhigh. There's no way to express this per-agent today.

Proposed API

---
name: development-agent
description: Implements GitHub issues end-to-end
model: opus
effort: xhigh
---

Accepted values should match the API: low, medium, high, xhigh, max.

Note: xhigh and max are Opus-only — if effort is set to one of these on a non-Opus model, Claude Code should either warn or fall back gracefully.

Current workaround

Only global effortLevel in settings.json is available. Per-agent effort is not configurable.

Related issues

#64706, #43083, #63599, #65531, #64033