docs(devicetree): use the ev_ retained Evidence id prefix in the example

The retained-reference example wrote the Evidence id as "ev:<digest>".
Retained ids are "ev_" followed by the digest (prefixedDigestSchema in
src/domain/digests.ts), and inspect_analysis_view rejects the colon form
with invalid_format. Use the same "ev_<digest>" placeholder as the other
guides.

Fixes #7612
Supersedes #7760

Co-Authored-By: gensokyo-dev <gensokyo-dev@users.noreply.github.com>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/devicetree.md

@@ -41,7 +41,7 @@ MCP `detail: summary` retains complete Evidence in the session. Follow its
 
 ```json
 {
-  "source": { "kind": "retained-evidence", "evidence_id": "ev:<digest>" },
+  "source": { "kind": "retained-evidence", "evidence_id": "ev_<digest>" },
   "view": {
     "kind": "devicetree",
     "collection": "properties",