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/tts/tts.go

@@ -5,9 +5,9 @@
 // PCM, headerless per the audio package; the voice sink + reference client
 // wrap it in a WAV at the disk edge.
 //
-// Per DESIGN.md § Voice pipeline (STT / TTS): piper is the production tts
+// Per docs/design.md § Voice pipeline (STT / TTS): piper is the production tts
 // (subprocess + espeak-ng, CPU-only on the ryzen box, driven by cmd/mavttsd);
-// the older silero pick is retired (DESIGN.md § Superseded, "named STT/TTS
+// the older silero pick is retired (docs/design.md § Superseded, "named STT/TTS
 // model picks"). A different voice is a model-file swap, not a code change.
 // The daemon wires one impl — Remote pointing at the worker socket if
 // configured, Stub otherwise.