feat(integration): add local harness for live contract tests

[Malomalsky]

What Changed

• Added scripts/integration-local.sh.
  • Starts local simplex-chat (if SIMPLEX_WS_URL is not provided)
  • Waits until websocket API is reachable
  • Runs integration tests with -tags=integration
  • Cleans up spawned process on exit
• Updated docs to include harness usage:
  • README.md
  • CONTRIBUTING.md
  • integration/README.md

Why

Running live integration tests required manual setup. This harness makes local contract test runs reproducible and faster for contributors.

How To Use

./scripts/integration-local.sh

Use existing websocket endpoint:

SIMPLEX_WS_URL=ws://localhost:5225 ./scripts/integration-local.sh

Validation

• bash -n scripts/integration-local.sh
• go test ./...
• go vet ./...
• ./scripts/check-compat.sh