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>

internal/stt/stt.go

@@ -18,9 +18,9 @@
 // returns a canned string the router + action path operate on); with a
 // worker socket configured, it wires Remote.
 //
-// Per DESIGN.md § Voice pipeline (STT / TTS): whisper.cpp (CGo, Vulkan) in
+// Per docs/design.md § Voice pipeline (STT / TTS): whisper.cpp (CGo, Vulkan) in
 // cmd/mavsttd is the production stt — the older faster-whisper/vosk picks are
-// retired (DESIGN.md § Superseded, "named STT/TTS model picks"). The
+// retired (docs/design.md § Superseded, "named STT/TTS model picks"). The
 // server-side stt module is the heavy multilingual path; the client's
 // wake-word + stage-0 command grammar (cmd/mavwaked) hits the router directly
 // and never crosses this seam. Today's Remote + Stub both return plain text