Add caching, shallow clones, and Gradle performance tuning

Optimizations added:
1. Shallow git clones (fetch-depth: 1) - Faster checkout
2. Enhanced Gradle caching:
   - Cache Gradle packages (~/.gradle/caches, ~/.gradle/wrapper)
   - Cache Gradle build cache (build-cache-* and .gradle directory)
3. Gradle performance tuning flags:
   - --build-cache: Use Gradle build cache
   - --parallel: Run tasks in parallel
   - --max-workers=4: Limit parallel workers
   - --no-daemon: Disable daemon for CI
   - -Dorg.gradle.jvmargs="-Xmx4g": Increase heap size
   - -Dkotlin.compiler.execution.strategy=in-process: Faster Kotlin compilation

Also removed TimingsListener from student/build.gradle (incompatible with future config cache support)

Author: hermannakos