docs: reorganize documentation into server/ and web/ directories

- docs: move server docs to docs/server/ and update all cross-references
- docs: move web-roadmap.md to docs/web/roadmap.md and create
  docs/web/decisions.md
- docs: update AGENTS.md with separate Server and Web sections

docs/README.md

@@ -1,8 +1,19 @@
 # Docs
 
+Documentation is organized by project. Use `server/` for the Go backend and `web/` for the browser client.
+
+## Server
+
 | File | Content |
 |------|---------|
-| `architecture.md` | Module layout, package boundaries |
-| `decisions.md` | Technical decisions (ADR) |
-| `roadmap.md` | Feature progress and status |
-| `development.md` | Build, test, debug workflow |
+| `server/architecture.md` | Module layout, package boundaries |
+| `server/decisions.md` | Technical decisions (ADR) |
+| `server/roadmap.md` | Feature progress and status |
+| `server/development.md` | Build, test, debug workflow |
+
+## Web
+
+| File | Content |
+|------|---------|
+| `web/decisions.md` | Technical decisions (ADR) |
+| `web/roadmap.md` | Product boundary, foundation, planned structure, and deferred dependencies |