docs/: move the design and working documents out of the repo root (CLAUDE.md and AGENTS.md stay, harnesses read them there)

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

client-core/src/lib.rs

@@ -1,5 +1,5 @@
 //! The app's pure logic, shared between the server and any Rust client --
-//! see `CLIENT_CORE.md` at the repo root for what lives here and what does
+//! see `docs/CLIENT_CORE.md` for what lives here and what does
 //! not yet.
 
 pub mod ansi;