docs: AGENTS.md and core architecture documentation

[tunahorse]

Summary

• add AGENTS.md for contributor guidance
• add core architecture documentation under docs/

Files

• AGENTS.md
• docs/core-agent-loop-architecture.md
• docs/mapping-layer.md
• docs/services-layer.md
• docs/tools-layer.md

[danwritecode]

I'd like to move to an idiomatic solution to this using doc comments and inner doc comments:

https://doc.rust-lang.org/rustdoc/index.html#outer-and-inner-documentation

We should utilize inner doc comments and then link to deeper docs if necessary in the docs dir. However i'd like to try to collapse everything into the lib.rs file where we can dump the inner doc comments and leave behind the docs dir entirely. i don't think it's that complex to warrant all of those pages. Lets reduce the verbosity of those docs too by like 50%, hit the high level shit and people can read the source code for the rest. I don't want to put into english the source code.

From opus:

Inner doc comments (//!) at the top of lib.rs become the crate's landing page on docs.rs. Use //! to document the enclosing item
  (crate/module) vs /// which documents the next item.

  #![doc = include_str!("../docs/file.md")] inlines an external markdown file as documentation, keeping long-form docs out of source
  files while still rendering them in cargo doc.