Improve interop-rules.md loading: trigger on diff content, not just file presence

The previous heuristic loaded interop rules only when both C# and native
files changed. This missed cases where a C#-only diff touches JNI/P/Invoke
boundary code (e.g., DllImport, [Register], JNIEnv, [MarshalAs],
[StructLayout]). Now triggers based on actual interop markers in the diff.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jonathanpeppers

.github/skills/android-reviewer/SKILL.md

@@ -72,7 +72,7 @@ Based on the file types identified in step 2, read the appropriate rule files fr
 - `references/csharp-rules.md` — When any `.cs` files changed. Covers nullable, async, error handling, performance, and code organization.
 - `references/msbuild-rules.md` — When `.targets`, `.props`, `.projitems`, or `.csproj` files changed, or when MSBuild task C# files changed (e.g., files under `src/Xamarin.Android.Build.Tasks/`).
 - `references/native-rules.md` — When `.c`, `.cpp`, `.h`, or `.hpp` files changed. Covers memory management, C++ best practices, symbol visibility, and platform-specific code.
-- `references/interop-rules.md` — When both C# and native files changed, or when P/Invoke or JNI boundary files changed (e.g., files under `src/Mono.Android/` that contain `DllImport` or files under `src/native/` that reference managed types).
+- `references/interop-rules.md` — When both C# and native files changed, or when the diff contains P/Invoke or JNI interop code (e.g., `DllImport`, `[Register]` attribute changes, `JNIEnv` calls, `[MarshalAs]`, `[StructLayout]`, or marshal-related changes in files under `src/Mono.Android/` or `src/native/`).
 - `references/testing-rules.md` — When test files changed (e.g., files under `tests/`, `**/Tests/`, or test project directories).
 - `references/security-rules.md` — When any code files changed (C#, C/C++, or MSBuild).