docs: rewrite README for a public audience

[adworzynski]

What

Phase 1 of taking EAaC public — rewrite the README for a stranger, not a maintainer. Docs-only,
no design impact.

• README top now leads with the thesis ("EA as deterministic, validated code; AI supplies
  only judgment") and a verified quick-look demo before the dev instructions, which move to
  the bottom. Adds license / CI / python badges and a contract diagram with P1/P2.
• Install documents pip install git+… via pipx/uvx (no PyPI — decided out of scope
  for now), with a @v0.1.0 version-pin example.
• ea pack fetch is wired into install + quick-look: fetching the method library (packs +
  presets) into ~/.cache/ea resolves offline, so the quick-look no longer needs
  EA_PACK_PATH/EA_PRESET_PATH exports.
• Fixes a latent inaccuracy: the old demo adopted startup-light, which is a test fixture, not
  a production preset → switched to lean-ux.
• examples/README notes the read-only verbs resolve from each example's materialized
  definitions/ (need nothing), with ea pack fetch . for authoring your own.

Proof

Docs-only — no src/ change, so no new tests. Every command documented was run against the
live CLI and returns what the README claims:

• ea pack fetch . → ok, fetches 6 packs + 3 presets into the cache; ea preset list then
  resolves them offline with no path vars.
• Build flow ea init → ea manifest adopt lean-ux → ea validate --all → all status: ok.
• Example poking (validate --all, type show, graph dependency, report coverage in
  examples/retailco-enterprise/repo) → all ok standalone, no packs on path.
• examples/payments-oaa/repo: ea validate --all → ok (exit 0), ea manifest show → ok.

No agent-facing behavior changed.

Notes

• The git+https://… install/clone/fetch commands and the @v0.1.0 pin assume the repo is
  public (Phase 2) — intentional; they're the documented public-facing commands.