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.

packages/subagent/tool-subagent/README.md

@@ -22,7 +22,7 @@ With `run_in_background: true`, the tool registers the parent-owned task before
 | `agentOptions` | Default child options, currently including `model`. |
 | `persona` | Per-child persona; requires provider `persona` capability. |
 | `toolFilter` | Per-child global-tool restriction; requires `toolFilter` capability. |
-| `maxDepth` | Absolute delegation-depth cap; requires `depthLimit` capability. |
+| `maxDepth` | Absolute delegation-depth cap, default `1` (`0` forbids delegation); a numeric cap requires the `depthLimit` capability and fails the mount without it. `'provider-managed'` sends no cap — for an out-of-process provider whose budget belongs to the child harness. A child AT the cap also loses this tool from its schema when the provider supports `toolFilter` (prompt-face hiding; the service still rejects on the execution face). |
 
 ## Concurrency