Add AI agent instructions (#27856)
### Description
Initial pass at some AI agent instructions for this repo.
Following some conventions:
- https://agents.md/
- https://agentskills.io/
Add repository instructions for AI agents covering:
- Build system phases (`--update`/`--build`/`--test`) and key flags
- High-level architecture (graph, optimizer, session, providers)
- C++ conventions (error macros, container types, span/string_view
preferences)
- Python conventions and testing patterns
- C API guidelines and PR expectations
Sourced from existing docs (`CONTRIBUTING.md`,
`docs/Coding_Conventions_and_Standards.md`, `docs/PR_Guidelines.md`,
`docs/C_API_Guidelines.md`) and codebase inspection.
Copilot put most of this together.
### Motivation and Context
Trying to help AI agents work better in this repo. Just getting some
initial instructions in place. We should refine it.
---------
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
