feat(tool-subagent): default maxDepth 1 with at-cap schema hiding

An omitted maxDepth meant unbounded recursion, and the shipped examples
shipped that default. maxDepth now defaults to 1; a numeric cap requires
the provider's depthLimit capability (the mount fails loud and points to
the explicit 'provider-managed' opt-out for out-of-process providers),
and a child AT the cap loses the delegating tool from its own schema via
the child toolFilter — prompt-face hiding on top of the execution-face
depth check. Examples pin maxDepth explicitly. The ACP snapshot harness
gains Scenario.childToolOmissions so a child session may legitimately
omit declared delegation tools from its pinned header and prompt;
affected subagent/workflow goldens are re-recorded.

docs/config-catalog.md

@@ -1127,8 +1127,7 @@ export interface Config {
   /**
    * Tool filter applied to every child. Filtered tools disappear from its
    * prompt and reject execution. Requires the provider's `toolFilter`
-   * capability; unknown names fail startup. Children otherwise see this tool,
-   * so deny it or set `maxDepth` to bound recursion.
+   * capability; unknown names fail startup.
    */
   toolFilter?: {
     /** Global tool names the child keeps; everything else is removed. */
@@ -1137,10 +1136,16 @@ export interface Config {
     deny?: string[]
   }
   /**
-   * Maximum child depth. Requires the provider's `depthLimit` capability and a
-   * non-negative safe integer. Omission is unbounded.
+   * Maximum child depth: a non-negative safe integer (default `1`; `0` forbids
+   * delegation entirely), or `'provider-managed'` to send no cap. A numeric cap
+   * requires the provider's `depthLimit` capability (mount fails loud
+   * otherwise), and a child AT the cap additionally loses this tool from its
+   * schema when the provider supports `toolFilter` — the prompt face of the
+   * budget; the service keeps rejecting on the execution face.
+   * `'provider-managed'` is for an out-of-process provider (ACP) whose
+   * recursion budget belongs to the child harness's own deployment.
    */
-  maxDepth?: number
+  maxDepth?: number | 'provider-managed'
 }
 ```