docs: simplify README for Codex Autopilot

[SignalLayerLabs]

What changed

• reduced the README from 354 to 218 lines
• moved installation and current Codex behavior ahead of theory and benchmark detail
• documented Autopilot, user intent, diagnostics, Earned Enforcement, and the exact enforcement boundary
• retained the public benchmark disclosures and privacy-profile contracts
• removed duplicated installation instructions and community-response narrative

Why

The previous README mixed product usage, theory, community history, and benchmark methodology. New
users had to read too far before learning how to install MARGINAL, what it can enforce, and what its
evidence does not prove.

Validation

• pytest -q — 571 passed
• ruff format --check .
• ruff check .
• mypy src/marginal
• deterministic Codex runtime check
• all local README links resolve
• README/publication contract tests pass