docs: state that Interlock installs from source, not PyPI

[FetchDoge]

What changed

• README Quickstart now states the verified fact: interlock is not published on PyPI, so pip install interlock does not install this package. The block adds the git clone step before pip install -e ".[test]" && pytest -q.
• Evidence line pins the 68-test count to the commit it ran on (6bc4b02).

Why

Marcus's profile README advertised pip install interlock. That name has no distribution on PyPI, so the claim fails for anyone who tries it, and an unclaimed name is a squatting risk. Docs only; no runtime changes.

How it was verified (2026-10-07, ~23:30 MT)

• curl https://pypi.org/pypi/interlock/json returns 404; https://pypi.org/simple/interlock/ returns 404; pip download --no-deps interlock reports "from versions: none".
• pyproject.toml declares name = "interlock", version 0.1.0 (from interlock.__version__).
• Fresh git clone of main at 6bc4b02, new venv, pip install -e ".[test]" && pytest -q: 68 passed.