feat(scripts): merge-mcp.sh — preserves laravel-boost, replaces brain-managed MCP

Uses jq --slurpfile pattern (NOT broken `.[1].mcpServers` form from plan):
  jq --slurpfile brain "$brain_mcp" \
     '.mcpServers = ((.mcpServers // {}) + $brain[0].mcpServers)' "$target"

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/fixtures/sample-claude-json.json

@@ -0,0 +1,14 @@
+{
+  "mcpServers": {
+    "laravel-boost": {
+      "command": "php",
+      "args": ["app/artisan", "boost:mcp"]
+    },
+    "magic": {
+      "type": "stdio",
+      "command": "cmd",
+      "args": ["/c", "npx", "-y", "@21st-dev/magic@latest", "API_KEY=OLD_VALUE"]
+    }
+  },
+  "preserveMe": "yes"
+}