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/kiwix/rewrite.go

@@ -31,7 +31,7 @@ type Completer interface {
 // or reply in Russian.
 //
 // Why the JSON wrapper: this model always thinks out loud and this llama-server
-// build ignores the thinking switch (see ROUTING-EVAL-31-07-2026.md). A bare
+// build ignores the thinking switch (see docs/evals/2026-07-31-routing.md). A bare
 // word-list grammar just captured the reasoning — every case came back as
 // "Let me analyze this request carefully". Demanding JSON, like routeGrammar and
 // responseGrammar already do, gives the reasoning nowhere to go.