docs: add Aurelius-to-Nerva full-stack workflow guide

[PAMulligan]

Summary

Adds docs/schema-to-api/aurelius-pipeline.md, the dedicated walkthrough for the /build-from-aurelius pipeline. The pipeline was referenced across the docs (CLAUDE.md, architecture, README) but had no guide of its own, so one of Nerva's most distinctive features — generating a matching backend from a frontend build spec — was effectively undocumented.

What it covers

The guide carries a single storefront example end to end (the same domain the command's own mapping table uses):

• How Aurelius generates build-spec.json — what the frontend records, with a concrete sample, and a callout distinguishing it from Nerva's own .claude/plans/build-spec.json (same filename, different file)
• What Nerva extracts — the Phase 1 analysis that infers models, auth strategy, pagination, and constraints
• How the OpenAPI spec is generated — the full generated OpenAPI 3.1 output, traced field-by-field back to the frontend
• Confirming the mapping — the Phase 3 component → endpoint table
• Handoff to /build-from-schema — into the standard 10-phase pipeline
• The typed API client — ./scripts/generate-client.sh (openapi-typescript + optional openapi-fetch runtime), the files it emits, and how the frontend imports it for end-to-end type safety
• Architecture diagram — an ASCII Aurelius ↔ Nerva data-flow diagram (matching the repo's diagram style; no Mermaid is used anywhere in the repo)
• Frontend-only data (design tokens) handling and a troubleshooting table

Wiring

• Linked from the main README.md — both the From Aurelius section and the Documentation table
• Linked from docs/schema-to-api/README.md via a new Where the Spec Comes From section

Notes

• Docs-only change; the affected CI job is the lychee link check. All internal link targets were verified to exist, the external Aurelius repo link returns 200, and inline-code backticks are balanced.

Closes #23

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