docs: improve README front door

[codeforester]

Summary

• Put a runnable source-checkout quickstart immediately after the product pitch.
• Define “local operating contract” in plain language and add a contents map near the top.
• Replace the overstated “90-Second Proof” label with a trust-conscious proof description.
• Mark the detailed product-layer command section as a skippable deep reference with focused-doc links.

Validation

• git diff --check
• /Users/rameshhp/.base.d/base/.venv/bin/python -m pytest tests/test_bootstrap_docs.py tests/test_contract_hardening.py -q — 17 passed

Fixes #1942