Add shared Blazor and cross-cutting contributor guidance

.github/instructions/blazor-components-guidance.instructions.md

@@ -0,0 +1,10 @@
+---
+description: Shared guidance for Components and JSInterop contributors
+applyTo: "src/Components/**,src/JSInterop/**"
+---
+
+For work in these areas, consult the relevant sections of the
+[Blazor and Components guidance](../../docs/BlazorComponentsGuidance.md).
+For `src/Components` work, follow the existing Components area instructions, including the
+implementation, sample, and E2E workflow. Generic `src/JSInterop`-only work does not inherit that
+workflow.

.github/instructions/cross-cutting-guidance.instructions.md

@@ -0,0 +1,8 @@
+---
+description: Shared cross-cutting guidance for source contributors
+applyTo: "src/**"
+---
+
+For source work, consult the relevant sections of the
+[cross-cutting guidance](../../docs/CrossCuttingGuidance.md).
+Use it as reference material within the requested task and existing area instructions.