docs: design brief for reusable LLM ingestion-pipeline security library

Brief-stage repo. docs/BRIEF.md defines a minimal, framework-agnostic
library that packages the write-time ingestion contract (sanitize ->
fence -> tool-less quarantined transform -> per-stage capability
isolation -> scan output before persist -> fail-secure) as reusable
code. Positioned honestly against query-time guardrails (LLM Guard,
NeMo, Rebuff, Vigil) with a prior-art verification log. Reference
implementation: claude-code-llm-wiki Stage B.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HPAmFyEVWbwvmSNVdXTu4d

README.md

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+# llm-ingestion-pipeline-security
+
+A reusable, minimal, dependency-light defensive layer for **LLM ingestion
+pipelines** — the write-time siblings of query-time chatbot guardrails.
+
+Where mature guardrails (LLM Guard, NeMo Guardrails, Rebuff, Vigil, …) sit
+between a user and a model at query time, this library hardens the other shape:
+untrusted content flowing through an LLM enrichment/summarization/extraction step
+into a **persisted, downstream-consumed artifact** (RAG corpus, knowledge base,
+wiki). It packages the architectural contract — sanitize → fence → tool-less
+quarantined transform → per-stage capability isolation → scan output before
+commit → fail-secure — as composable, framework-agnostic code.
+
+**Status:** brief / pre-implementation. Start with the design brief:
+
+- [Design brief](docs/BRIEF.md) — what this repo should contain and why.
+
+The contract is extracted from a working reference implementation (the
+`claude-code-llm-wiki` Stage B enrichment pipeline).