docs: fix Chat and Tools section accuracy in HELP.md

- Copy button appears on all messages (user + assistant), not just assistant
- Delete requires a confirm step — clarified in description
- Model tag position is below-left for assistant messages, not bottom-right
- Tool calls render as expandable per-call cards, not a single ⚡ N summary note
- help.html: default-open sections updated to Getting Started / Chat / Sessions / Model Registry

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cortex/static/help.html

@@ -198,7 +198,7 @@
     }
 
     // ── Load all three tabs in parallel ─────────────────────────────
-    const UI_OPEN = new Set(['Header Controls', 'Chat', 'Sessions', 'Notes']);
+    const UI_OPEN = new Set(['Getting Started', 'Chat', 'Sessions', 'Model Registry']);
 
     async function loadAll() {
       // UI Guide