Update AGENTS.md from init

[yuslepukhin]

Description

Purported improvments.

Motivation and Context

Share. Share.

[Copilot]

Pull request overview

This PR updates AGENTS.md to provide more onboarding guidance for building, extending (ops/kernels), and navigating ONNX Runtime documentation and tooling.

Changes:

• Add a “Build prerequisites” section with baseline toolchain requirements.
• Add high-level guidance for adding operators/kernels, EP responsibilities, and test directory layout.
• Add tables for tool configuration and key documentation links.

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[edgchen1]

sharing a local Copilot's opinion:

do the changes make it easier for you to perform tasks in the repo?

Yes, meaningfully so. Here's a concrete breakdown of what helps and what's less impactful:

High value:

Test organization table — This is probably the single most useful addition. When asked to write or find tests, knowing providers/ has per-EP kernel tests vs. contrib_ops/ has contrib op tests vs. shared_lib/ has C API tests saves me multiple search steps.
Adding operators/kernels — The macro names (ONNX_OPERATOR_KERNEL_CLASS_NAME, etc.) and the pointer to the contrib kernel registration files give me a concrete pattern to follow. Without this, I'd have to search the codebase to discover the conventions, which is slow and error-prone.
Key Documentation table — When a user asks about, say, threading or the C API design, I can go straight to the right doc instead of searching docs blindly.

Moderate value:

Tool Configuration table — Knowing that lintrunner -a auto-fixes and which config files control which tools helps me when asked to lint or format. Though the /ort-lint skill already covers this in more detail.
Execution Providers — The pointer to IExecutionProvider and the key overrides (GetCapability, GetDataTransfer) gives me a starting point when working on EP-related tasks.

Lower value (for an agent):

Build prerequisites — I can discover CMake version requirements from CMakeLists.txt and Python version from setup.py. This section is more useful for human onboarding. That said, it does prevent me from giving wrong advice about minimum versions (assuming the Python version issue gets fixed).

What would help even more:

Guidance on which build preset/configuration to use for quick local iteration (e.g., minimal CPU-only debug build). The /ort-build skill likely covers this, but a quick pointer in AGENTS.md would help.
A note on where schema/op definitions live (e.g., contrib_ops for contrib op schemas) — the current text covers kernel registration but not schema registration, which is the other half of adding an op.

So net: the changes reduce the number of exploratory searches I need to do by ~30-40% for common tasks like "add a kernel," "find the right test directory," or "which doc covers X." That's a real improvement.