Define and exercise a minimal distributed-system behavior (v0.2.0)

## Goal

Move beyond toolchain and execution correctness by exercising a *single* distributed-system behavior appropriate for an edge agent.

This issue is intentionally scoped to **one minimal proving example**.

## Motivation

v0.1.0 successfully establishes:
- ARM64 cross-compilation
- runtime correctness under QEMU
- reproducible CI workflows

What is still missing is a concrete demonstration of reasoning about an edge agent *as a constrained distributed system*, rather than as a
standalone program.

## Proposed scope (choose one)

One of the following is sufficient to close this issue:

- **Lifecycle driven by an external stimulus**
- e.g. message → state transition
- **Failure / reconnect behavior**
- e.g. simulated message bus outage → recovery
- **Explicit coordination contract**
- e.g. heartbeat, liveness check, or command acknowledgement

## Non-goals

- No hardware integration
- No firmware or device drivers
- No production-grade messaging guarantees
- No full async refactor unless required by the chosen example

## Acceptance criteria

- Behavior is observable (logs or output)
- Behavior is exercised in CI (native or QEMU)
- Code remains explicit and small
- README updated to reflect the added capability

## Versioning

Target: **v0.2.0**

Start by inspecting the existing edge-agent behavior, CI workflows, QEMU execution path, and README. Choose one proposed behavior, keep the implementation explicit and small, then verify observable logs or output in native or QEMU CI and document the capability in the README.

Written by the indexing model from the issue text.