chore: add chatd ARCHITECTURE.md and mention it in AGENTS.md (#26478)

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.

AGENTS.md

@@ -85,6 +85,10 @@ instructions focused on guardrails that agents should see immediately.
   [OAuth2 Development Guide](.claude/docs/OAUTH2.md). OAuth2 endpoints must
   use RFC-compliant errors such as `writeOAuth2Error(...)`, and public
   endpoints that need system access should use `dbauthz.AsSystemRestricted`.
+- **Chatd**: consult [Chatd Architecture](coderd/x/chatd/ARCHITECTURE.md) to
+  understand the architecture of the chatd subsystem. If you update the
+  chatd subsystem in ways that affect the architecture, you must update the
+  architecture document.
 - **API design**: Follow the API guardrails in
   [Development Workflows and Guidelines](.claude/docs/WORKFLOWS.md),
   including swagger annotations for new public HTTP endpoints.