agent: trace runs as OpenTelemetry spans

[asim]

Goal

Make agent runs observable as first-class runtime work by mapping RunInfo events, tool calls, delegation, and failures into OpenTelemetry spans.

Scope

• Add additive instrumentation around agent run execution without changing public agent APIs.
• Correlate run IDs, step/tool/delegation events, status, and provider failures with existing trace context where available.
• Keep default behavior no-op when tracing is not configured.
• Add deterministic tests or harness coverage that does not require provider secrets.

Acceptance criteria

• Agent run traces include run ID, agent name, model/provider when known, step/tool/delegation events, status, duration, and error attributes.
• micro inspect/run history breadcrumbs can point developers at trace correlation data when tracing is enabled.
• go test ./... covers the instrumentation path with a mock tracer/exporter.
• No breaking public API changes or required new runtime dependency defaults.