chore(mcp): flip .mcp.json to npx @webclaw/mcp; gitignore registry tokens

The launcher is published (@webclaw/mcp@0.6.16), so the in-repo MCP config
now uses the canonical npx form instead of a local ~/.webclaw binary path.
Also ignore the mcp-publisher credential files so they can never be
committed from packages/create-webclaw/.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.mcp.json

@@ -1,7 +1,8 @@
 {
   "mcpServers": {
     "webclaw": {
-      "command": "~/.webclaw/webclaw-mcp"
+      "command": "npx",
+      "args": ["-y", "@webclaw/mcp"]
     }
   }
 }