Replaces the Mintlify setup (mint.json, docs.json, snippets) with a
standalone Fumadocs app in docs/: Next.js app router, meta.json nav per
section, its own bun install. Run with `just dev-docs`.
Content pass alongside the migration: core architecture, data model,
CLI, and networking pages rewritten around the daemon/source-store/
library split, new product-direction.mdx defining the product boundary,
new drive-catalog plan, and the physical-drives plan updated to
reference it. Whitepaper page retired.
