README as onboarding

[emafriedrich]

Rewrites the README as an onboarding page instead of a methodology summary.

• Leads with the problem: agents infer product rules from implementation, invent what is missing, and cannot tell current from historical knowledge — and documentation rots because nothing checks it.
• What KDE does about it: computable current truth, explicit precedence, bounded agent retrieval, agents propose / humans promote, integrity in CI, and backfill for knowledge that lives only in code.
• Field evidence, anonymized: the six-day snapshot rot, the wrong accepted decision caught by diffing knowledge against code, and one backfilled domain (22 rules, 3 specs, 2 unimplemented decisions, 3 defects).
• Brownfield-first quick start: install, backfill <domain>, ask the agent. Greenfield seeding stays as the second path.
• How it works (domains and artifacts, precedence, lifecycle commands, CI gates), when to use it, honest limits, prior art, and a map of the repository.

No behavior change.