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>

frontend/src/components/GuideReader.svelte

@@ -245,7 +245,7 @@
             >
               {#if ROLE_ICON[t.tab]}<span class="tab-ic">{ROLE_ICON[t.tab]}</span>{/if}
               {t.tab}
-              {#if t.audience === 'internal'}<span class="tab-int">internal</span>{/if}
+              {#if t.audience !== 'customer'}<span class="tab-int">{t.audience}</span>{/if}
             </button>
           {/each}
         </div>