Trace agent run event kinds

[asim]

Summary

• Add a stable agent.event.kind OpenTelemetry attribute to agent run timeline span events so backends can query run/model/tool/checkpoint/resume/done events without parsing event names.
• Tighten the existing no-secret agent OTel test helper to assert the event kind attribute is present alongside RunInfo correlation.

Closes #3525
Closes #3854

Testing

• go test ./agent
• go build ./...
• go test ./... (fails in sandbox: loopback targets forbidden in grpc client/transport tests)
• golangci-lint run ./...