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/router/eval/llmrouter_test.go

@@ -39,7 +39,7 @@ import (
 // Re-measured with everything else held equal, thinking off scores exactly the
 // same, case for case — and a direct probe shows this llama-server build ignores
 // enable_thinking / reasoning_budget for this model anyway, so there was nothing
-// to turn off. Full write-up in ROUTING-EVAL-31-07-2026.md (Vikunja #376).
+// to turn off. Full write-up in docs/evals/2026-07-31-routing.md (Vikunja #376).
 func TestLLMRouterBaseline(t *testing.T) {
 	base := os.Getenv("MAVEN_LLM_URL")
 	if base == "" {