init: readme 작성 하네스 설계

docs/architecture.md

@@ -0,0 +1,25 @@
+# Architecture
+
+README Harness has one public build function and one read-only review boundary.
+
+```text
+user request + repository
+          │
+          ▼
+       writer
+          │ draft + compact facts
+          ▼
+build_readme ── scan / Markdown / command / path / link / secret checks
+          │
+          ├─ facts.json
+          ├─ validation.json
+          ├─ README.generated.md
+          └─ README.patch
+          │
+          ▼
+read-only reviewer ── review.json ── one revision at most
+```
+
+`README.generated.md` is normal GitHub Markdown. Fact bindings live in
+`facts.json` and address a natural document block by section, block number, and
+text hash. The target README changes only through the explicit `apply` command.