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/config/config.go

@@ -649,7 +649,7 @@ type VoiceConfig struct {
 	// LLMRouter — route with the resident model instead of the embedding
 	// classifier. On by default since Vikunja #320.
 	//
-	// Measured on the held-out fixture (ROUTING-EVAL-31-07-2026.md): 63.2% of
+	// Measured on the held-out fixture (docs/evals/2026-07-31-routing.md): 63.2% of
 	// intents right against the classifier's 50.0%, and no route errors. It
 	// costs about 1s per turn instead of 30ms.
 	//