Add the AGENTS.md

[eap]

Add an AGENTS.md file. This file sets ground rules for how agents interact with spack-stack and gives documentation pointers and agent-specific instructions.

This file was primarily generated by claude using notes from a long-running install and debug session.

Motivations

• Constrain unwanted agent behaviors; no autonomous pull requests, tool attributions, destructive behavior.
• Give agents advice on common tasks; install, debug, site updates.
• Agent-specific advice on managing spack commands; executing and following commands with overwhelming output or long runtimes like concretize and install

[mathomp4]

Might be interesting to look at mom-ocean/MOM6#1699

In that @adcroft made a sort of symlink or reference nest of various markdown files to support a few different agents (CLAUDE.md, AGENTS.md, .github/copilot-instructions.md, etc.).

This might not be needed anymore?