fix: resolve build/index.js via ${CLAUDE_PLUGIN_ROOT} so plugin works from any cwd

Claude Code resolves the .mcp.json args path against the user's cwd, not
the plugin's install directory. Without this fix, the MCP server only
starts when Claude is launched from inside a clone of this repo.

Co-authored-by: Nathan Kettles <natekettles@users.noreply.github.com>

.mcp.json

@@ -2,7 +2,7 @@
   "mcpServers": {
     "apple-mail": {
       "command": "node",
-      "args": ["build/index.js"]
+      "args": ["${CLAUDE_PLUGIN_ROOT}/build/index.js"]
     }
   }
 }