Closes
[CODAGT-610](https://linear.app/codercom/issue/CODAGT-610/add-an-architecturemd-file-to-chatd).
Adds an ARCHITECTURE.md file which describes the architecture of the
chatd subsystem. It's meant for reading by both humans, who would like
to understand chatd better, and agents. It's an edited version of the
chatd stabilization RFC.
