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/phraser/eval/eval.go

@@ -11,7 +11,7 @@
 // length test that a human can read and disagree with. A score here is a claim
 // about measurable properties, not about whether a sentence is good.
 //
-// DESIGN.md § "Rules decide, LLM phrases" is why there is no send/veto signal
+// docs/design.md § "Rules decide, LLM phrases" is why there is no send/veto signal
 // anywhere in this package: the rule already decided she speaks. The phraser
 // only words it, so a nudge the model refuses to write is a failure, never a
 // legitimate outcome.
@@ -38,7 +38,7 @@ var fixtureJSON []byte
 const SchemaVersion = 1
 
 // Case — one nudge situation, as a real tick would present it. The fields are
-// the (rule, severity, context) input DESIGN.md names, flattened to JSON.
+// the (rule, severity, context) input docs/design.md names, flattened to JSON.
 //
 // WantAny is the on-topic contract: at least one of these lowercased fragments
 // must appear in the message. A water nudge that never mentions water is a