[FEATURE]: Support project-scope agent-agnostic MCP configuration at `.agents/mcp/servers.json`

[NiJeTi]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

As stated in Agent-Agnostic Repository Guide.

Canonical format:

{
  "figma": {
    "type": "http",
    "url": "https://mcp.figma.com/mcp"
  },
  "stacks": {
    "type": "http",
    "url": "https://stacksmcp.int.stackoverflow.net/mcp"
  }
}