docs: subagent frontmatter `effort` field is implemented but undocumented

Summary

Claude Code 2.1.258 parses an effort field in custom subagent definition files (.claude/agents/*.md, ~/.claude/agents/*.md), but the sub-agents documentation does not list it, so users assume the field does not exist (or that a value they wrote is inert).

Evidence

The agent-definition schema embedded in the 2.1.258 binary contains:

effort: enum(["low","medium","high","xhigh","max"]) | int, optional
  "Reasoning effort level for this agent. Either a named level or an integer"

It sits alongside the other frontmatter fields (maxTurns, background, memory, permissionMode, observer). The Agent tool description also states that each agent type's "model, reasoning effort, and tools come from its definition (.claude/agents/*.md frontmatter or SDK agents)".

The official docs, however:

• https://code.claude.com/docs/en/sub-agents — the frontmatter field table lists only name, description, tools, model, permissionMode, skills, memory, isolation. No effort.
• https://code.claude.com/docs/en/model-config — documents effortLevel, modelSettings.<model>.effortLevel, CLAUDE_CODE_EFFORT_LEVEL and /effort, but says nothing about subagents (neither the frontmatter field nor whether a subagent inherits the parent session's effort).

This is the follow-up to #43083 (closed 2026-08-17), whose later comments show users still unsure whether the feature exists or what the default is.

Requested change

1. Add effort to the sub-agents frontmatter table: accepted values (low/medium/high/xhigh/max or an integer), and an example.
2. State the precedence and default: what a subagent uses when effort is omitted (parent session effort? modelSettings for its model? a fixed default?).
3. Optionally cross-link from the model-config page.

Environment

• Claude Code 2.1.258, macOS