a2a: governance-aware inbound — correlation, tags, spans and typed refusals on served tasks #225

Summary

What donkey.serve() adds over a plain A2A server: every call arriving over
A2A gets the same treatment as an outgoing one.

Acceptance criteria

• An inbound task establishes a run scope with a correlation ID, derived
  from the caller's ID when present and generated when not.
• Cost tags apply to work done on behalf of an inbound task.
• One span per inbound task, linked to the spans of downstream calls it makes.
• A downstream refusal surfaces to the A2A caller as a well-formed A2A
  error, not a leaked stack trace.

Cross-surface impact

• area:a2a + area:telemetry + area:refusals: the change.
• Docs site: an inbound-governance page.

Out of scope

Inbound policy enforcement — that is the gateway's job, not the SDK's.