Format Markdown with prettier and move documents out of the root (closes #411)

REPO_POLICIES.md names prettier as the Markdown formatter and keeps documents out of the root. script/fmt and script/fmt-check now run prettier over all Markdown (4-space indents, proseWrap always) besides gofmt, with node and yarn installed by script/bootstrap as in sneak/prompts. All Markdown is reformatted with no text change. ARCHITECTURE.md and TODO.md move to docs/, config.example.yml to configs/, and every reference follows.

Model: opus-5-5

yarn.lock

@@ -0,0 +1,8 @@
+# THIS IS AN AUTOGENERATED FILE. DO NOT EDIT THIS FILE DIRECTLY.
+# yarn lockfile v1
+
+
+prettier@3.8.1:
+  version "3.8.1"
+  resolved "https://registry.yarnpkg.com/prettier/-/prettier-3.8.1.tgz#edf48977cf991558f4fcbd8a3ba6015ba2a3a173"
+  integrity sha512-UOnG6LftzbdaHZcKoPFtOcCKztrQ57WkHDeRD9t/PTQtmT0NHSeWWepj6pS0z/N7+08BHFDQVUrfmfMRcZwbMg==