#17788: fix: correct config path in sessions_spawn error hint (v2)

## Summary Fix incorrect config path in error message. ## Changes - Correct config path: `agents.list[].subagents.allowAgents` (not `agents.defaults.subagents.allowAgents`) ## Testing - pnpm build - pnpm check ## AI-assisted Yes Fixes #17390 <!-- greptile_comment --> <h3>Greptile Summary</h3> Fixes the config path in the `sessions_spawn` error hint from the incorrect `agents.defaults.subagents.allowAgents` to the correct `agents.list[].subagents.allowAgents`. The `allowAgents` setting is a per-agent config defined on individual agent entries in `agents.list[]` (see `AgentConfig.subagents.allowAgents` in `src/config/types.agents.ts`), not on `agents.defaults`. The corrected path is consistent with all existing documentation (`docs/tools/subagents.md`, `docs/concepts/session-tool.md`, `docs/tools/index.md`). - Corrected config path in the error hint shown when `agentId` is not in the allowlist and the allowlist is empty - No functional behavior change beyond the error message content <h3>Confidence Score: 5/5</h3> - This PR is safe to merge — it only corrects a string literal in an error message. - The change is a single-line fix to a config path in an error hint string. No logic, control flow, or behavior is modified. The corrected path (`agents.list[].subagents.allowAgents`) is verified against the type definitions and documentation. - No files require special attention. _{Last reviewed commit: e10a5cf} <!-- greptile_other_comments_section --> <!-- /greptile_comment -->