Document instructions for editable intalls in AGENTS.md

### What kind of documentation issue is this?

Missing documentation

### Affected page

AGENTS.md

### Describe the issue

I've been using spec-kit to work on scaffold introduction. During the process, the LLM tries to install things editably, but fails at that because of the repository layout and the need to use ``--no-deps`` as things depend on each other.

### Suggested improvement

Add instructions to do editable installs to AGENTS.md

Start with AGENTS.md and inspect the repository layout and existing installation guidance. Document how to perform editable installs in this repository, including when --no-deps is needed because packages depend on each other. Done means the instructions let the scaffold workflow install the relevant packages editably without the reported failure.

Written by the indexing model from the issue text.