Add Streamable HTTP + OAuth support for remote MCP servers

[pranavp311]

Summary

Graff 0.0.201 only supports spawning stdio MCP servers from .mcp.json. It cannot connect directly to remote Streamable HTTP MCP servers with OAuth, such as Mobbin (https://api.mobbin.com/mcp).

Evidence

• graff mcp add persists only {command,args,env}.
• src/mcp.zig describes and implements a stdio-only client by spawning child processes.
• Mobbin requires Streamable HTTP plus OAuth and is natively supported by Codex via:
  • codex mcp add mobbin --url https://api.mobbin.com/mcp
  • codex mcp login mobbin
• In a Graff session, I had to invoke a separate authenticated codex exec process as a bridge to call Mobbin MCP.

Expected

Support config entries such as:

{
  "mcpServers": {
    "mobbin": {
      "url": "https://api.mobbin.com/mcp"
    }
  }
}

with MCP Streamable HTTP negotiation, OAuth discovery/login/token persistence, and bounded request/response handling. Ideally:

graff mcp add mobbin --url https://api.mobbin.com/mcp
graff mcp login mobbin

The CLI should distinguish stdio and HTTP transports and report unsupported auth/capability states clearly.