docs(readme): rewrite for the newcomer, led by the compliance engine

[eaitbrahim]

Closes #281

The README was a 261-line operator runbook — right for the author's future self, wrong for a
stranger deciding whether to spend an afternoon here.

The new first screen

1. What keel is, led by the compliance engine — 19 deterministic rails, attested screening
   that fails closed, §65.4 qabd as an executable check; the trading agent is the reference
   implementation on top, not the headline.
2. The honest result, stated by us first — no shipped rule family net-positive at the
   taker fee actually paid; viable intersection empty under production-faithful execution,
   linking docs/experiments/2026-08-13-restated-under-a-production-faithful-engine.md.
3. The not-a-fatwa-engine boundary (from Phase 6, kept verbatim).
4. Try it in five minutes — see below.

Quickstart, verified end-to-end on a clean clone (that's why it says keel rules seed,
not keel init: a clone has the tracked config.yaml and init refuses to overwrite it):

git clone … && cd keel
uv sync --all-extras --dev
cp .env.example .env          # free read-only CDP key
uv run keel rules seed
uv run keel fetch
uv run keel simulate --years 1 --skip-within-cap

Two honest caveats are in the text because they were measured: keel fetch without a CDP
key dies in AuthenticationError (tested), and keel simulate on default rules reports
TRAIN MORE with the failing gates named — presented as the engine working, because it is.

Rest of the structure: condensed How-it-works (rules, rails, screening, confirm/autonomy,
ships-inert, the tighter-stop→larger-position mechanic), an architecture sketch naming the
three load-bearing places (keel/execution/guards.py, keel/agent.py,
packages/keel-broker-* port + entry point), a documentation map, and an ending that routes
questions to Discussions (now enabled) and contributions to CONTRIBUTING.md.

Moved, not deleted: deploying/upgrading releases, the paper-vs-live table and its
interactions, and "how much money moves" now live in docs/operator-runbook.md (Part 2,
verbatim in substance).

Tests (tests/test_readme.py, red first): honest result above the fold + cited record
exists; quickstart commands verbatim incl. the CDP caveat and paper path; the three
architecture locations named; README ends with Discussions/CONTRIBUTING routing; operator
content present in the runbook and absent from the README (the anti-runnable-regression
guard).

Gates: pytest 2794 passed / 1 skipped, ruff check clean, mypy clean.