Add Makefile with common dev commands

[crtahlin]

Sub-issue of #114 (developer experience review).

Task

Create a Makefile with targets:

• help — show available targets
• setup — create venv, install editable with test+blockchain deps
• test / test-unit — run unit tests (no integration)
• test-integration — run integration tests (requires live backends)
• lint — ruff check . && ruff format --check .
• format — ruff check --fix . && ruff format .
• clean — remove build artifacts, caches, egg-info

Why

Currently developers need to remember pytest flags and ruff commands. A Makefile provides discoverable, single-command access to common tasks.

[miles-on-nightshift]

Triage: classification: enhancement, priority: priority-low

Makefile is a developer convenience. Nice-to-have.

[miles-on-nightshift]

Triage: feature / priority: low

Makefile is a developer convenience. Not blocking anything.

Labels to add: enhancement, priority-low