feat: Add Notification hook handler for formatting idle notifications

Format raw JSON IPC messages from workers/teammates into user-friendly
display instead of showing raw JSON to users.

Changes:
- Add notification.py hook handler to hookify plugin that formats
  idle_notification, status_update, and progress_update messages
- Update hookify hooks.json to include Notification event handler
- Add Pattern 11 documentation for formatting teammate idle notifications
- Add format-idle-notification.sh example script

Raw JSON input like:
  {"type":"idle_notification","from":"worker-1","timestamp":"..."}

Now displays as:
  ⏺ worker-1
    ⎿ Status is idle

Slack thread: https://anthropic.slack.com/archives/C07VBSHV7EV/p1765785226571409?thread_ts=1765776251.343939&cid=C07VBSHV7EV

plugins/plugin-dev/skills/hook-development/examples/format-idle-notification.sh

@@ -0,0 +1,50 @@
+#!/bin/bash
+# Example: Format teammate idle notification
+#
+# This script demonstrates how to format raw JSON idle notifications
+# into user-friendly display format.
+#
+# Usage: echo '{"type":"idle_notification","from":"worker-1","timestamp":"..."}' | ./format-idle-notification.sh
+
+set -euo pipefail
+
+# Read JSON from stdin
+input=$(cat)
+
+# Parse notification type
+notification_type=$(echo "$input" | jq -r '.type // empty' 2>/dev/null || echo "")
+
+if [[ "$notification_type" == "idle_notification" ]]; then
+  # Extract fields
+  worker_name=$(echo "$input" | jq -r '.from // "worker"')
+  timestamp=$(echo "$input" | jq -r '.timestamp // empty')
+
+  # Format timestamp if present
+  time_str=""
+  if [[ -n "$timestamp" ]]; then
+    # Try to format the timestamp
+    time_str=$(date -d "$timestamp" '+%H:%M:%S' 2>/dev/null || echo "")
+  fi
+
+  # Output formatted notification using recommended format:
+  # ⏺ worker-1
+  #   ⎿ Status is idle
+  echo "⏺ $worker_name"
+  if [[ -n "$time_str" ]]; then
+    echo "  ⎿ Status is idle ($time_str)"
+  else
+    echo "  ⎿ Status is idle"
+  fi
+
+  # Output JSON for hook system
+  if [[ -n "$time_str" ]]; then
+    jq -n --arg msg "⏺ $worker_name\n  ⎿ Status is idle ($time_str)" \
+      '{"systemMessage": $msg}'
+  else
+    jq -n --arg msg "⏺ $worker_name\n  ⎿ Status is idle" \
+      '{"systemMessage": $msg}'
+  fi
+else
+  # Not an idle notification, pass through
+  echo "$input"
+fi