docs: restructure README around first verified task

[0thernet]

Summary

• lead with KB's concrete purpose, outcome, and local evidence boundary
• give one tested first task with prerequisites, commands, and observable filesystem result
• keep the complete command and adapter reference after the compact site-selected introduction
• link repository guidance to the shared Hraness README contract
• update the exact package-size fixture for the reviewed README bytes

Verification

• bun run check (1,196 tests, 0 failures)
• first task executed in a fresh temporary vault
• focused README renderer review in the consuming Hraness site
• independent final documentation review