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>

Makefile

@@ -78,7 +78,7 @@ deps-go:
 	done
 	$(GO) version
 
-# fmt-check fails if any file needs gofmt. DESIGN.md has always said `make
+# fmt-check fails if any file needs gofmt. docs/design.md has always said `make
 # test` gates on gofmt and vet; it did not, so nine files quietly drifted.
 # Run `gofmt -w` on whatever this prints.
 fmt-check:
@@ -191,7 +191,7 @@ deps-piper:
 # multilingual-e5-small: an asymmetric retrieval model. It is trained to match
 # a short question against a longer passage, which is what note recall is.
 # The quantized file is the one we download, deploy and measure — see
-# RECALL-EVAL-31-07-2026.md.
+# docs/evals/2026-07-31-recall.md.
 EMBEDDER_DIR := $(shell pwd)/models/embedder/multilingual-e5-small
 EMBEDDER_MODEL_URL := https://huggingface.co/Xenova/multilingual-e5-small/resolve/main/onnx/model_quantized.onnx
 EMBEDDER_TOKENIZER_URL := https://huggingface.co/Xenova/multilingual-e5-small/resolve/main/tokenizer.json