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

.prettierignore

@@ -0,0 +1,2 @@
+node_modules/
+yarn.lock