docs(readme): add 'How keel works' section

[eaitbrahim]

Adds a How keel works section to the root README, filling a real gap: the operating model (how orders actually get placed, why there's no manual trade command, DCA's calendar cadence) was previously only reconstructable from the design spec, runbooks, and code docstrings.

Covers, in a screenful:

• The agent loop — poll → each live rule's detect() → rails → preview → confirm/autonomy gate → place_order → log.
• Rules-only, no manual order command — every order is rule output; the four rule kinds and that DCA is the one on a calendar cadence (the others fire on real market setups).
• The un-overridable rails — allowlist, spend/exposure/concentration caps, kill-switch, drawdown & edge-decay breakers, no-martingale/no-stop-widening, feed/balance/attestation checks.
• Confirm vs autonomy — changes who is asked, never what's allowed.
• Halal + ships inert — long-only spot, no riba; nothing trades until a rule is live, attested, funded, and confirmed.

Verified against guards.py, agent.py, executor.py, and the runbooks so it doesn't drift. Docs-only.

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