Document source exports, subpaths, and Convex requirements

[0thernet]

Summary

• README.md:12-23 explains the four source subpaths, no root import, Convex peer requirement, and reference/reauthentication paths; verified package.json:11-27 and backend/index.ts:21-43.
• Part of a portfolio documentation pass that applies Stripe and Vercel docs patterns: a complete first result, task-based navigation, exact defaults, and troubleshooting checked against source.

Test plan

• host-run --mode=shared --lane=compute --label=docs-relay-final -- bun run check: Exit 0 after waiting 155.7 seconds; typecheck and 99 crypto/wire/Convex tests passed across 6 files, 0 failed.
• Changed commands, defaults, and claims checked against the owning source; independent review findings addressed
• Required CI passes on this head

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