feat: add check-inbox.sh with marker-based change detection

Only outputs when there are new messages since last check.
Uses .inbox-last-id marker file to track highest seen message ID.

Co-Authored-By: Virgil <virgil@lethean.io>

claude/core/scripts/check-inbox.sh

@@ -0,0 +1,66 @@
+#!/bin/bash
+# Check for new inbox messages since last check.
+# Silent if no new messages. Only outputs when there's something new.
+
+MARKER_FILE="$HOME/Code/host-uk/core/.core/workspace/.inbox-last-id"
+BRAIN_KEY=$(cat "$HOME/.claude/brain.key" 2>/dev/null | tr -d '\n')
+
+if [ -z "$BRAIN_KEY" ]; then
+    exit 0
+fi
+
+# Get agent name
+HOSTNAME=$(hostname | tr '[:upper:]' '[:lower:]')
+if echo "$HOSTNAME" | grep -qE "snider|studio|mac"; then
+    AGENT="cladius"
+else
+    AGENT="charon"
+fi
+
+# Fetch inbox
+RESPONSE=$(curl -sf -H "Authorization: Bearer $BRAIN_KEY" \
+    "https://api.lthn.sh/v1/messages/inbox?agent=$AGENT" 2>/dev/null)
+
+if [ -z "$RESPONSE" ]; then
+    exit 0
+fi
+
+# Get last seen ID
+LAST_ID=0
+if [ -f "$MARKER_FILE" ]; then
+    LAST_ID=$(cat "$MARKER_FILE" | tr -d '\n')
+fi
+
+# Check for new messages
+NEW=$(echo "$RESPONSE" | python3 -c "
+import sys, json
+data = json.load(sys.stdin)
+msgs = data.get('messages') or data.get('data') or []
+last_id = int(${LAST_ID})
+new_msgs = [m for m in msgs if m.get('id', 0) > last_id]
+if not new_msgs:
+    sys.exit(0)
+# Update marker to highest ID
+max_id = max(m['id'] for m in new_msgs)
+print(f'NEW_ID={max_id}')
+for m in new_msgs:
+    print(f\"  {m.get('from_agent', m.get('from', '?'))}: {m.get('subject', '(no subject)')}\")
+" 2>/dev/null)
+
+if [ -z "$NEW" ]; then
+    exit 0
+fi
+
+# Extract new max ID and update marker
+NEW_ID=$(echo "$NEW" | grep "^NEW_ID=" | cut -d= -f2)
+if [ -n "$NEW_ID" ]; then
+    echo "$NEW_ID" > "$MARKER_FILE"
+fi
+
+# Output for Claude
+DETAILS=$(echo "$NEW" | grep -v "^NEW_ID=")
+COUNT=$(echo "$DETAILS" | wc -l | tr -d ' ')
+
+echo "New inbox: $COUNT message(s)"
+echo "$DETAILS"
+echo "Use agent_inbox to read full messages."