#12808: docs(subagents): clarify allowAgents is per-agent only

## Summary Clarifies that `allowAgents` is a per-agent setting and is not supported under `agents.defaults.subagents`. ## What changed - Added a note in `docs/tools/subagents.md` under Cross-Agent Spawning: - `allowAgents` belongs under `agents.list[].subagents` - `agents.defaults.subagents` does not accept `allowAgents` - Added the same clarification in `docs/concepts/session-tool.md` in the `sessions_spawn` allowlist section. ## Why Users naturally place `allowAgents` under `agents.defaults.subagents` (because other subagent defaults live there), which causes config validation errors. This makes the docs explicit at the two places users check first. Closes #11982 <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> Docs-only PR clarifying that `allowAgents` is configured per-agent under `agents.list[].subagents`, and is not a valid key under `agents.defaults.subagents`. The same clarification is added in the `sessions_spawn` allowlist section of the Session Tools doc and as a note under the Sub-Agents tool “Cross-Agent Spawning” section to prevent common config validation errors when users place `allowAgents` in the global defaults block. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge; it only adds clear, consistent documentation notes. - Changes are limited to two docs files and accurately describe existing config semantics (per-agent `allowAgents` vs global `agents.defaults.subagents`). No behavioral or code changes, and the added notes are consistent with the surrounding sections and examples. - No files require special attention <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} **Context used:** - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) <!-- /greptile_comment --> ## Contributing checklist - [x] AI-assisted: yes (prepared with OpenClaw assistant, model: `openai-codex/gpt-5.3-codex`) - [x] Testing level: lightly tested (docs-only change; link/build/test not run for this docs PR) - [x] Confirmed understanding of change and impact