add AGENTS.md file

[orbeckst]

Add a file to tell coding agents how to work on the project

• follow coding standards (see toml file and developer docs (TO BE ADDED))
• update docs when adding or changing code
• add tests for new features
• do not change existing tests without asking for permission
• document design decisions in the Plans/ directory (follow examples there); add timestamps to filename and in document

[orbeckst]

You can have a look at https://github.com/Becksteinlab/ConfoState/tree/main/.cursor/rules for the cursor-specific rules. They could just be combined in an AGENTS file (perhaps except the one about the mamba env which is a bit too specialized (and which I perhaps should remove again))