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/messaging/discord.md

@@ -383,6 +383,26 @@ display:
   tool_progress_command: true
 ```
 
+## Interactive Model Picker
+
+Send `/model` with no arguments in a Discord channel to open a dropdown-based model picker:
+
+1. **Provider selection** — a Select dropdown showing available providers (up to 25).
+2. **Model selection** — a second dropdown with models for the chosen provider (up to 25).
+
+The picker times out after 120 seconds. Only authorized users (those in `DISCORD_ALLOWED_USERS`) can interact with it. If you know the model name, type `/model <name>` directly.
+
+## Native Slash Commands for Skills
+
+Hermes automatically registers installed skills as **native Discord Application Commands**. This means skills appear in Discord's autocomplete `/` menu alongside built-in commands.
+
+- Each skill becomes a Discord slash command (e.g., `/code-review`, `/ascii-art`)
+- Skills accept an optional `args` string parameter
+- Discord has a limit of 100 application commands per bot — if you have more skills than available slots, extra skills are skipped with a warning in the logs
+- Skills are registered during bot startup alongside built-in commands like `/model`, `/reset`, and `/background`
+
+No extra configuration is needed — any skill installed via `hermes skills install` is automatically registered as a Discord slash command on the next gateway restart.
+
 ## Home Channel
 
 You can designate a "home channel" where the bot sends proactive messages (such as cron job output, reminders, and notifications). There are two ways to set it: