Custom-agent reference documents reasoningEffort, but the working frontmatter key is reasoning-effort #4963

Describe the bug

The Copilot CLI custom-agent reference documents reasoningEffort as a frontmatter field. In Copilot CLI 1.0.88, that spelling did not apply the configured effort when I selected the agent with /agent. Changing only the field name to reasoning-effort made it work.

This matches the syntax reported in a comment on #2904. The reference currently leads users to configure a field that does not work in this case.

Affected version

GitHub Copilot CLI 1.0.88; macOS arm64.

Steps to reproduce the behavior

1. Define a custom agent with reasoningEffort: "low" in its YAML frontmatter.
2. Select it with /agent and observe that the configured effort does not take effect.
3. Change only that key to reasoning-effort: "low", reload the agent, and select it again.
4. Observe that the configured effort now takes effect.

Expected behavior

Update the custom-agent frontmatter reference to document reasoning-effort as the working key, including its description and precedence examples. If reasoningEffort is also intended to be supported, document both spellings and fix the camelCase spelling separately; it should not be presented as working while it is not.

Additional context

The issue is the documented frontmatter spelling, not a request to add per-agent reasoning effort: that capability was addressed in #2904.