core: trace executor skill discovery

[anp-oai]

Why

Make it easier to measure the performance of different parts of skill loading.

What

• Add spans for step-context capture, world-state construction, executor catalog snapshot/root loading, and environment skill loading.
• Record the discovered environment skill count.
• Trace outbound exec-server requests with client kind and RPC method fields.
• Update trace propagation tests to assert that requests keep the parent trace id while creating their own child span.