feat(guide): dev audience tier + GUIDE_SERVED_AUDIENCES env cap (prod = customer,internal)

4-tier ladder customer<internal<dev<bcos. Re-tagged Architect/Testing/Claude +
platform/technical from internal->dev; Processes stays bcos. Backend visible()
enforces a per-instance served-audiences cap at the API. bcos.dev/DLX serve all;
production sets GUIDE_SERVED_AUDIENCES=customer,internal so dev/bcos never leave
dev. Verified: capped instance returns only Customer+Internal, 404s dev/bcos.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/main/resources/content/recon/recon-rules/testing.md

@@ -3,7 +3,7 @@ module: recon.recon-rules
 title: Recon Rules
 tab: Testing
 order: 35
-audience: internal
+audience: dev
 ---
 
 Frontend test checklist for **Recon Rules** (`recon.bcos.dev`). Click through in the UI and mark each row Pass or Fail.