Add SBOM, symbols, traces, and VEX files for CVE-2022-21661 SQLi case

- Created CycloneDX and SPDX SBOM files for both reachable and unreachable images.
- Added symbols.json detailing function entry and sink points in the WordPress code.
- Included runtime traces for function calls in both reachable and unreachable scenarios.
- Developed OpenVEX files indicating vulnerability status and justification for both cases.
- Updated README for evaluator harness to guide integration with scanner output.

src/Signals/StellaOps.Signals/Persistence/ICallgraphRepository.cs

@@ -7,7 +7,9 @@ namespace StellaOps.Signals.Persistence;
 /// <summary>
 /// Persists normalized callgraphs.
 /// </summary>
-public interface ICallgraphRepository
-{
-    Task<CallgraphDocument> UpsertAsync(CallgraphDocument document, CancellationToken cancellationToken);
-}
+public interface ICallgraphRepository
+{
+    Task<CallgraphDocument> UpsertAsync(CallgraphDocument document, CancellationToken cancellationToken);
+
+    Task<CallgraphDocument?> GetByIdAsync(string id, CancellationToken cancellationToken);
+}