Add AGENTS.md with Cursor Cloud development environment instructions

[SimonBaars]

Summary

Adds AGENTS.md with Cursor Cloud-specific development instructions for the Instant Massive Structures Mod (IMSM). Also makes gradlew executable.

What's included

• Java 8 requirement documentation (Gradle 2.7 + ForgeGradle 2.2 are incompatible with Java 9+)
• Development commands table (build, test, check, run client, setup workspace)
• Known caveats for cloud/VM environments (SSL transient errors, no audio, software rendering, etc.)
• Made gradlew executable (was missing execute permission)

Environment verification

The development environment was fully verified:

• Build: ./gradlew build completes successfully
• Tests: ./gradlew test and ./gradlew check pass
• Run: Minecraft 1.10.2 launches with the IMSM mod loaded (4 mods active)

1.11.2 Port

A separate mc-1.11 branch has been created with a full port to Minecraft 1.11.2 with Forge 13.20.1.2386. See the mc-1.11 branch for details.