[MINOR][DOC] Add AGENTS.md with guidelines for AI coding agents

[nookcreed]

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What changes are proposed in this pull request?

Adds AGENTS.md — a top-level file that provides guidelines for AI coding agents (e.g. Claude Code, Cursor, Copilot)
working in this repository.

The file covers:

• Pointers to key developer documentation (build guides, contributing guide, C++ style guide)
• Build order requirements (native backend must be built before Maven, with concrete commands for both Velox and
  ClickHouse backends)
• Pre-commit checklist: Scala formatting, C++ formatting, and license header checks
• PR title convention summarizing the [GLUTEN-][BACKEND] pattern with a quick reference table
• Testing instructions for Java/Scala unit tests and both native backends
• AI tooling disclosure reminder as required by the ASF Generative Tooling Guidance

This follows the emerging convention (google/A2A, anthropics/claude-code) of providing an AGENTS.md file so AI
assistants can orient themselves to a project's conventions without reading the full contributor docs.

How was this patch tested?

Documentation-only change. No code was modified. Manually verified that all referenced scripts and docs exist in the
repository.

Was this patch authored or co-authored using generative AI tooling?

Generated-by: Claude claude-sonnet-4-6