Add an edge-cloud capability contract extension sample

## Context

I proposed an optional edge-cloud capability contract extension in a2aproject/A2A#2076, following the extension governance process.

## Proposed sample

Add an experimental extension sample under `extensions/edge-cloud-capability-contract/v1` containing:

- a normative extension specification;
- JSON Schemas for Agent Card capability claims, per-request execution requirements, and response execution receipts;
- a dependency-free fail-closed reference matcher;
- tests covering edge/offline/residency selection and rejection for unsatisfied latency, region, attestation, verification, or idempotency requirements.

The sample uses existing `AgentExtension.params`, extension activation, and Message/Artifact metadata. It does not change core A2A types, add a competing protocol, or treat declarations as trust anchors.