chore: add AGENTS.md with Cursor Cloud dev environment instructions

[barnett]

Fixes

Adds AGENTS.md with Cursor Cloud-specific development environment instructions for the Trolley Java SDK.

What this PR does

Adds an AGENTS.md file documenting:

• Build and test commands (mvn compile, mvn test, mvn package)
• JAVA_HOME requirement for the javadoc plugin
• Integration test credential requirements (.env with ACCESS_KEY / SECRET_KEY)
• JVM config notes for PowerMock compatibility

Evidence of working environment

build_and_hello_world.log

Build verification:

• mvn compile — 60 source files compiled successfully
• mvn test-compile — test sources compiled successfully
• mvn package -DskipTests -Dgpg.skip=true — JAR, sources JAR, and javadoc JAR built
• mvn test — all 18 integration tests ran (fail with AuthenticationException without API credentials — expected)
• Hello world program exercising the SDK's Configuration, Gateway, and HTTP layer — all working

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have titled the PR appropriately
• I have updated my branch with the main branch

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