docs: tier the tree by lifetime, so staleness shows in the path (V-446)

Seventeen markdown files at the repo root, twelve of them dated one-shot
reports sitting next to CLAUDE.md. That is why stale docs read as
current: nothing in the path said which was which.

Root now keeps CLAUDE.md and AGENTS.md. Living docs move under docs/
and carry a Last verified line. Dated measurements move to docs/evals/
ISO-prefixed, and are never edited after the day, so a newer number is
a new file. The senior review moves to docs/archive/.

Every reference was rewritten across markdown, Go comments, the Makefile
and the recall fixture. The touched Go packages still build.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

cmd/mavenclient/main.go

@@ -1,6 +1,6 @@
 // Package main is mavenclient — maven's reference client.
 //
-// Per DESIGN.md § Voice pipeline (STT / TTS): capture lives on the client;
+// Per docs/design.md § Voice pipeline (STT / TTS): capture lives on the client;
 // the server transcribes + synthesises on demand. The PC client runs the
 // wake-word / VAD gate (cmd/mavwaked) and ships ONE clean audio blob per
 // utterance on activation. The server never owns a mic.

cmd/mavweb/main.go

@@ -512,7 +512,7 @@ func main() {
 
 	// /tools — the authed enable surface. maven proposes acts she can't run;
 	// this page is where a human reviews and enables them (proposed→enabled).
-	// Enabling is the boundary-moving act (DESIGN.md § Tool registration —
+	// Enabling is the boundary-moving act (docs/design.md § Tool registration —
 	// drafting is suggest, enabling is act), so it lives ONLY here,
 	// behind wg+nginx+auth — never the voice/chat path.
 	mux.HandleFunc("/tools", func(w http.ResponseWriter, r *http.Request) {