feat: add portable agent instructions to repo

[gknapp1]

Adds AI agent development instructions for improving agent-assistant development consistency:

• AGENTS.md
• docs/agents subagent definitions
• .codex directory with subagent wrappers (defers to instructions in docs/agents)

Closes #165

[gknapp1]

@liamnwhite1, I might be jumping the gun a bit with this but I've been experimenting with subagent delegation, so I'm going to leave this in draft while I test it out some more. I think you might have had tested some of this already for the GitHub subagents, so curious what you think about this implementation and how it interplays with that functionality.

[liamnwhite1]

@gknapp1 This looks absolutely amazing! I think the agent Skills I've been working on will integrate perfectly with this. I'm going to rebase on this branch and test out integrating those skills with these agents. Also, I think we should meet to draw some of these things out on a whiteboard because I've been thinking about a lot of this in terms of what a development workflow looks like and for each step what an agent would look like and what skills it would need. I'd love to see what your and Sam's thoughts are on the overall structure!