Introduce Vexer platform scaffolding and enrich Concelier merge

src/StellaOps.Vexer.Connectors.RedHat.CSAF/AGENTS.md

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+# AGENTS
+## Role
+Connector for Red Hat CSAF VEX feeds, fetching provider metadata, CSAF documents, and projecting them into raw storage for normalization.
+## Scope
+- Discovery via `/.well-known/csaf/provider-metadata.json`, scheduling windows, and ETag-aware HTTP fetches.
+- Mapping Red Hat CSAF specifics (product tree aliases, RHSA identifiers, revision history) into raw documents.
+- Emitting structured telemetry and resume markers for incremental pulls.
+- Supplying Red Hat-specific trust overrides and provenance hints to normalization.
+## Participants
+- Worker schedules pulls using this connector; WebService triggers ad-hoc runs.
+- CSAF normalizer consumes fetched documents to produce claims.
+- Policy/consensus rely on Red Hat trust metadata captured here.
+## Interfaces & contracts
+- Implements `IVexConnector` with Red Hat-specific options (parallelism, token auth if configured).
+- Uses abstractions from `StellaOps.Vexer.Connectors.Abstractions` for HTTP/resume helpers.
+## In/Out of scope
+In: data acquisition, HTTP retries, raw document persistence, provider metadata population.
+Out: normalization, storage internals, attestation, general connector abstractions (covered elsewhere).
+## Observability & security expectations
+- Log provider metadata URL, revision ids, fetch durations; redact tokens.
+- Emit counters for documents fetched, skipped (304), quarantined.
+## Tests
+- Connector harness tests (mock HTTP) and resume regression cases will live in `../StellaOps.Vexer.Connectors.RedHat.CSAF.Tests`.