Tracks nested skill execution depth per process tree.

Skills can be configured to include the skills toolset tag, which gives them access to the skill runner tool. This enables skill-to-skill calls.

To prevent runaway recursion, we track depth across nested calls and refuse execution once a maximum is reached.

Depth is scoped to the current process tree via Services.Globals, so concurrent skill chains (e.g., parallel delegate steps) each maintain their own independent counter.