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/delivery/routing_table_test.go

@@ -9,7 +9,7 @@ import (
 	"github.com/kami/maven/internal/store"
 )
 
-// This file walks every cell of the DESIGN.md § "Delivery / channel routing"
+// This file walks every cell of the docs/design.md § "Delivery / channel routing"
 // table, once as the pure table and once through the dispatcher, so a change
 // to either side has to break a named cell.
 //
@@ -205,7 +205,7 @@ func TestAwayChannelsGetMinimalBody(t *testing.T) {
 // An empty Summary no longer means "send the whole body" — it means a short
 // generic line — so the old expectation here was wrong as well as duplicated.
 
-// TestCareAwayDropIsRecorded — DESIGN.md's drop is a decision ("a missed water
+// TestCareAwayDropIsRecorded — docs/design.md's drop is a decision ("a missed water
 // nudge is noise, a missed backup failure isn't"), so it should be visible
 // rather than vanish. Today drop is a bare `continue`: no nudge row, no outbox
 // attempt, no log — nothing an operator can see afterwards. now it leaves a