observability.md credits Arcp.Runtime ActivitySource with spans it never starts

[nficano]

Category: docs Severity: minor
Location: docs/guides/observability.md:44-44

What

ArcpDiagnostics.Runtime (src/Arcp.Core/Tracing/ArcpDiagnostics.cs:46) is never used to start an Activity; only ArcpDiagnostics.Transport starts spans (src/Arcp.Otel/ArcpTracing.cs:33,70). No dispatch/agent-run spans are produced.

Evidence

| `Arcp.Runtime`    | Runtime-internal spans (dispatch, agent run).      |

Proposed fix

Note that the Arcp.Runtime source currently starts no spans, or add the runtime spans.

Acceptance criteria

• The doc credits the Runtime ActivitySource only with spans the code actually starts.

[nficano]

Fixed in #77 (merged).