Add applyTo front-matter to all .github/instructions files

Without the applyTo field, Copilot never auto-applies these instructions
based on file context. Add glob patterns so the right instructions are
included automatically:
- test-instructions.md: **/*Test.kt, **/*IT.kt
- implement-instructions.md: **/*.kt, **/*.java
- refactor-instructions.md: **/*.kt, **/*.java
- document-instructions.md: **/*.md
- review-instructions.md: **/*.kt, **/*.java

Co-authored-by: afranken <763000+afranken@users.noreply.github.com>

Author: Copilot

.github/instructions/document-instructions.md

@@ -1,3 +1,6 @@
+---
+applyTo: "**/*.md"
+---
 # Documentation Instructions for S3Mock
 
 Read `AGENTS.md` (root + relevant module) before updating documentation.

.github/instructions/implement-instructions.md

@@ -1,3 +1,6 @@
+---
+applyTo: "**/*.kt, **/*.java"
+---
 # Implementation Instructions for S3Mock
 
 Read `AGENTS.md` (root + relevant module) before making any changes.

.github/instructions/refactor-instructions.md

@@ -1,3 +1,6 @@
+---
+applyTo: "**/*.kt, **/*.java"
+---
 # Refactor Instructions for S3Mock
 
 Read `AGENTS.md` (root + relevant module) before refactoring.

.github/instructions/review-instructions.md

@@ -1,3 +1,6 @@
+---
+applyTo: "**/*.kt, **/*.java"
+---
 # Review Instructions for S3Mock
 
 Read `AGENTS.md` (root + relevant module) before reviewing code.

.github/instructions/test-instructions.md

@@ -1,3 +1,6 @@
+---
+applyTo: "**/*Test.kt, **/*IT.kt"
+---
 # Test Instructions for S3Mock
 
 Read `AGENTS.md` (root + relevant module) before writing tests.