[FLINK-39881] Add AGENTS.md and generative AI tooling disclosure to PR template

[spuru9]

What is the purpose of the change

This pull request adopts the Apache Flink AI-assisted contribution guidelines for the Flink Kubernetes Operator repository. It does two things:

1. Adds a "Was generative AI tooling used to co-author this PR?" section to the repository's pull request template, mirroring the section already present in apache/flink.
2. Adds a root AGENTS.md providing guidance for AI coding agents working with this codebase, modeled on apache/flink's AGENTS.md and adapted to the operator.

This is a follow-up to FLINK-39477, which adopted AI-assisted contribution guidelines for the main Flink repository. That ticket was closed with a note that other Flink-project owned repositories should create related follow-ups — this PR is the follow-up for the Kubernetes Operator.

Brief change log

• Append the "Was generative AI tooling used to co-author this PR?" section to .github/PULL_REQUEST_TEMPLATE.md, including the disclosure checkbox, the Generated-by: line, and a link to the ASF Generative Tooling Guidance.
• Add a root AGENTS.md covering prerequisites, build/test commands, repository structure, the operator/observer/reconciler architecture, common change patterns, coding/testing standards, and contribution boundaries — adapted from apache/flink's AGENTS.md for this repository (Java 17, plain mvn, no Scala, CRDs as the public API surface).
• Reference AGENTS.md from the PR template's generative-AI section ("See AGENTS.md for the full guidance"), matching apache/flink's wording now that the file exists.

Verifying this change

This change is a trivial rework / code cleanup without any test coverage (PR-template / documentation only).

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

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changes to the CustomResourceDescriptors: no
• Core observer or reconciler logic that is regularly executed: no

Documentation

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

---

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

• Yes (please specify the tool below)

Generated-by: Claude Code (Claude Opus 4.8)

[spuru9]

cc: @Dennis-Mircea @gyfora PTAL!

[Dennis-Mircea]

The change looks good on its own. One thing worth noting: the Flink core project pairs this with an AGENTS.md file.

Either way, I'd defer to the PMC level on the timing and rollout.

[spuru9]

@1996fanrui @gyfora Can you take a look at this, Thanks.

[spuru9]

Also added a root AGENTS.md (adapted from apache/flink) and restored the template's "See AGENTS.md" reference now that the file exists; updated the title/description to match.