docs: comprehensive docs audit — cover 13 features from last week's PRs (#5815)

Cover documentation gaps found by auditing all 50+ merged PRs from the past week:

tools-reference.md:
- Fix stale tool count (47→46, 11→10 browser tools) after browser_close removal
- Document notify_on_complete parameter in terminal tool description

telegram.md:
- Add Interactive Model Picker section (inline keyboard, provider/model drill-down)

discord.md:
- Add Interactive Model Picker section (Select dropdowns, 120s timeout)
- Add Native Slash Commands for Skills section (auto-registration at startup)

signal.md:
- Expand Attachments section with outgoing media delivery (send_image_file,
  send_voice, send_video, send_document via MEDIA: tags)

webhooks.md:
- Document {__raw__} special template token for full payload access
- Document Forum Topic Delivery via message_thread_id in deliver_extra

slack.md:
- Fix stale/misleading thread reply docs — thread replies no longer require
  @mention when bot has active session (3 locations updated)

security.md:
- Add cross-session isolation (layer 6) and input sanitization (layer 7)
  to security layers overview

feishu.md:
- Add WebSocket Tuning section (ws_reconnect_interval, ws_ping_interval)
- Add Per-Group Access Control section (group_rules with 5 policy types)

credential-pools.md:
- Add Delegation & Subagent Sharing section

delegation.md:
- Update key properties to mention credential pool inheritance

providers.md:
- Add Z.AI Endpoint Auto-Detection note
- Add xAI (Grok) Prompt Caching section

skills-catalog.md:
- Add p5js to creative skills category

website/docs/user-guide/features/delegation.md

@@ -184,7 +184,7 @@ Delegation has a **depth limit of 2** — a parent (depth 0) can spawn children
 - Subagents **cannot** call: `delegate_task`, `clarify`, `memory`, `send_message`, `execute_code`
 - **Interrupt propagation** — interrupting the parent interrupts all active children
 - Only the final summary enters the parent's context, keeping token usage efficient
-- Subagents inherit the parent's **API key and provider configuration**
+- Subagents inherit the parent's **API key, provider configuration, and credential pool** (enabling key rotation on rate limits)
 
 ## Delegation vs execute_code