docs(architecture): make section pages canonical

AGENTS.md

@@ -141,7 +141,7 @@ Keep the tools standard-library-only where practical; if a third-party import is
 
 ## Architecture documentation
 
-[`Docs/Architecture/`](Docs/Architecture/) is the arc42 documentation: twelve numbered section files, `about-arc42.md`, and the combined [`architecture.md`](Docs/Architecture/architecture.md). **Markdown is canonical** — the `.adoc` sources were removed on 2026-11-08.
+[`Docs/Architecture/`](Docs/Architecture/) is the arc42 documentation: [`README.md`](Docs/Architecture/README.md) provides the overview and links to twelve numbered section files; `about-arc42.md` records the template attribution. **Markdown is canonical** — the `.adoc` sources were removed on 2026-11-08.
 
 Keep it current **in the same change as the implementation**, not in a later documentation pass. When work alters any of the following, edit the matching section:
 
@@ -160,7 +160,7 @@ Rules:
 
 - **Describe what exists.** No aspirational architecture: anything planned belongs in `ROADMAP.md` and `TODO.md`. Every structural claim names the file, script or flag that implements it.
 - **ADRs are append-only.** Never rewrite an accepted decision; supersede it with a new ADR that says what it replaces and why.
-- **Keep the combined document in sync.** `architecture.md` mirrors the section files; when a section changes, update the combined document too.
+- **Keep the index current.** `Docs/Architecture/README.md` links to the canonical section pages; update it when the structure or section list changes.
 - **Images** live in `Docs/Architecture/images/` and are referenced, not duplicated.
 - **Verify:** all Markdown must pass `prettier --check` (config in `.prettierrc`), and a command you quote must still run.