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/signal.md

@@ -147,13 +147,26 @@ Group access is controlled by the `SIGNAL_GROUP_ALLOWED_USERS` env var:
 
 ### Attachments
 
-The adapter supports sending and receiving:
+The adapter supports sending and receiving media in both directions.
+
+**Incoming** (user → agent):
 
 - **Images** — PNG, JPEG, GIF, WebP (auto-detected via magic bytes)
 - **Audio** — MP3, OGG, WAV, M4A (voice messages transcribed if Whisper is configured)
 - **Documents** — PDF, ZIP, and other file types
 
-Attachment size limit: **100 MB**.
+**Outgoing** (agent → user):
+
+The agent can send media files via `MEDIA:` tags in responses. The following delivery methods are supported:
+
+- **Images** — `send_image_file` sends PNG, JPEG, GIF, WebP as native Signal attachments
+- **Voice** — `send_voice` sends audio files (OGG, MP3, WAV, M4A, AAC) as attachments
+- **Video** — `send_video` sends MP4 video files
+- **Documents** — `send_document` sends any file type (PDF, ZIP, etc.)
+
+All outgoing media goes through Signal's standard attachment API. Unlike some platforms, Signal does not distinguish between voice messages and file attachments at the protocol level.
+
+Attachment size limit: **100 MB** (both directions).
 
 ### Typing Indicators