docs: restructure README — problem/solution/who above the fold

Lead with the problem (agents forget everything), solution (L3
remembers), and audience (agents on Lightning). Tools grouped by
use case: Memory, Intelligence, Payment Safety, Trust, Marketplace.
Added semantic search section. Cleaned up flow for cold readers.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: singularityjason