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/auth/tier.go

@@ -1,7 +1,7 @@
 // Package auth is maven's authority layer — the 4-layer cascade and the
 // "surface caps authority" invariant.
 //
-// Spec contract (from DESIGN.md § Auth):
+// Spec contract (from docs/design.md § Auth):
 //
 //	a cascade, not a pick-one — each layer answers a different question:
 //