General's built-in subagent deny makes experimental.subagent_depth ineffective for General #60961

Description

experimental.subagent_depth controls how deep subagents can nest, but the built-in agent plugin also denies the subagent tool to General (packages/core/src/plugin/agent.ts). Raising the depth therefore does nothing for General, the default worker most delegations use. Users also have to find a second setting and allow subagent for General in their agent config.

The deny was added on 2026-06-30 (c26f6f9, "deny subagent tool for default subagents"), two weeks before the depth limit (#37124). At the default depth of 1 it is redundant, since the depth check already stops a subagent from starting another one. Its only remaining effect at depth 1 is hiding the tool, which avoids the failing calls described in #53356.

Proposal

1. Fix subagent: tool remains advertised at the maximum session depth #53356: don't offer the subagent tool when the session is at the depth limit.
2. Then remove the subagent deny from General, so subagent_depth is the single control.

The default behavior stays the same: at depth 1, only the primary agent delegates. Raising the depth then works without a second setting, and users who never want General to delegate can still deny it in their config.

Related: #53356, #59826, #50539.

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