feature: add GET /echo that returns the query message

## Problem
We need a trivial request→response round-trip to prove the coding agent can add a parameterized route and a test.

## Outcome
`GET /echo?message=hello` returns HTTP 200 with JSON `{"message":"hello"}`. Missing `message` returns 400.

## Spec reference
spec/features/example-happy-path.feature

## Acceptance criteria
Given the Node service in `src/`
When a client calls GET /echo?message=hello
Then the response status is 200
And the body is JSON with message equal to hello

Given the Node service in `src/`
When a client calls GET /echo with no message query param
Then the response status is 400
And the body is JSON with an error field

## Constraints
- Keep the change small under `src/`
- Open a **draft** pull request; do not merge
- Update `docs/pr-evidence.md`
- Follow AGENTS.md