Implement Advisory Canonicalization and Backfill Migration

- Added AdvisoryCanonicalizer for canonicalizing advisory identifiers.
- Created EnsureAdvisoryCanonicalKeyBackfillMigration to populate advisory_key and links in advisory_raw documents.
- Introduced FileSurfaceManifestStore for managing surface manifests with file system backing.
- Developed ISurfaceManifestReader and ISurfaceManifestWriter interfaces for reading and writing manifests.
- Implemented SurfaceManifestPathBuilder for constructing paths and URIs for surface manifests.
- Added tests for FileSurfaceManifestStore to ensure correct functionality and deterministic behavior.
- Updated documentation for new features and migration steps.

src/Scanner/__Libraries/StellaOps.Scanner.Surface.FS/ISurfaceManifestWriter.cs

@@ -0,0 +1,11 @@
+namespace StellaOps.Scanner.Surface.FS;
+
+/// <summary>
+/// Publishes manifest documents to the configured manifest store.
+/// </summary>
+public interface ISurfaceManifestWriter
+{
+    Task<SurfaceManifestPublishResult> PublishAsync(
+        SurfaceManifestDocument document,
+        CancellationToken cancellationToken = default);
+}