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;

client-core/src/transcript_cache.rs

@@ -1,7 +1,7 @@
 //! This phone's copy of the transcripts it has already been sent, so
 //! reopening a session does not download it again. Ported from
-//! `app/.../TranscriptCache.kt`; see `TRANSCRIPT_CACHE.md` at the repo root
-//! for the design and `CLIENT_CORE.md` for how this file corresponds to it.
+//! `app/.../TranscriptCache.kt`; see `docs/TRANSCRIPT_CACHE.md`
+//! for the design and `docs/CLIENT_CORE.md` for how this file corresponds to it.
 //!
 //! What is stored is the server's own JSON for one event per line, in
 //! transcript order. Reading the cache means running the same [`seq_of`]