What is the purpose of the change

Make the Flink codebase accessible to AI coding agents and establish lightweight guidelines for AI-assisted contributions.

Brief change log

• Added AGENTS.md at the repository root with guidelines for AI coding agents: prerequisites, build/test commands, repository structure, architecture boundaries, common change patterns, coding standards, testing standards, commit conventions, and boundaries
• Added a dedicated AI disclosure section to the PR template with a checkbox and commented-out Generated-by tag, aligned with the ASF Generative Tooling Guidance

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

Was generative AI tooling used to co-author this PR?

• Yes: Claude Code 1.0.33 (Claude Opus 4.6, 1M context)