#8361: docs: Add subagent configuration and allowlist guide

This documentation guides users on how to configure the policy in to resolve the error. It provides clear examples for allowing all or specific agents. This was based on a real-world use case during a security audit. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> Adds a new documentation page describing how to resolve the `sessions_spawn` “agentId is not allowed” forbidden error by configuring sub-agent spawning permissions in the OpenClaw config. The guide introduces an allowlist concept with examples for allowing any sub-agent vs restricting to specific agent ids, plus a short troubleshooting section. This overlaps with (and should align to) the existing `/tools/subagents` documentation, which already defines the `agents.list[].subagents.allowAgents` semantics used by `sessions_spawn`. <h3>Confidence Score: 3/5</h3> - Mostly safe to merge, but the examples contain at least one copy/paste-breaking snippet and likely schema drift with existing docs. - This PR only adds documentation, so runtime impact is low; however, the guide is intended to fix a real error and currently includes an invalid JSON example and introduces `allowAny` semantics that conflict with the existing documented schema (`allowAgents: ["*"]`). Those issues could cause user confusion and failed configuration changes. - docs/concepts/subagent-configuration.md <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} **Context used:** - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->