Add AGENTS.md file for agentic coding

[joshbainbridge]

We have agreed to add dedicated support for those that would like to leverage agentic coding.

As an initial start, we should add agent rules, via an AGENTS.md file.

It may be best to follow the lead of OpenImageIO:

AcademySoftwareFoundation/OpenImageIO#5206

Time and consideration on the best approach has been spent there that we can capture.

Most of the changes there map across and make sense. However the following notes:

• .agents/skills/prepare-patch-release not to include as this is specific to OpenImageIO.
• .agents/skills/release-notes-update not to include as this is specific to OpenImageIO.
• AGENTS.md will need to be written by us for OpenQMC. Care should be taken over this.
• We should add a clear policy outline in the GOVERNANCE.md file or elsewhere on how contributions using agentic tools are to be tagged. Linux Kernel has a clear policy on this here we may want to also adopt. Another useful reference would be the Ghostty project by Mitchell Hashimoto.

[lgritz]

You may be interested also in this https://github.com/AcademySoftwareFoundation/OpenImageIO/blob/main/docs/dev/AI_Policy.md
which we had added in an earlier PR than the one you linked to.

[joshbainbridge]

I am indeed, thanks @lgritz.