Introduce Vexer platform scaffolding and enrich Concelier merge

src/StellaOps.Vexer.Formats.CSAF/AGENTS.md

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+# AGENTS
+## Role
+Normalize CSAF VEX profile documents into Vexer claims and provide CSAF export adapters.
+## Scope
+- CSAF ingestion helpers: provider metadata parsing, document revision handling, vulnerability/action mappings.
+- Normalizer implementation fulfilling `INormalizer` for CSAF sources (Red Hat, Cisco, SUSE, MSRC, Oracle, Ubuntu).
+- Export adapters producing CSAF-compliant output slices from consensus data.
+- Schema/version compatibility checks (CSAF 2.0 profile validation).
+## Participants
+- Connectors deliver raw CSAF documents to this module for normalization.
+- Export module leverages adapters when producing CSAF exports.
+- Policy engine consumes normalized justification/status fields for gating.
+## Interfaces & contracts
+- Parser/normalizer classes, helper utilities for `product_tree`, `vulnerabilities`, and `notes`.
+- Export writer interfaces for per-provider/per-product CSAF packaging.
+## In/Out of scope
+In: CSAF parsing/normalization/export, schema validation, mapping to canonical claims.
+Out: HTTP fetching (connectors), storage persistence, attestation logic.
+## Observability & security expectations
+- Emit structured diagnostics when CSAF documents fail schema validation, including source URI and revision.
+- Provide counters for normalization outcomes (status distribution, justification coverage).
+## Tests
+- Fixture-driven parsing/export tests will live in `../StellaOps.Vexer.Formats.CSAF.Tests` using real CSAF samples.