feat: add stdio MCP server support for client-side tool execution

[pmukeshreddy]

Summary

Implements #150 - adds support for connecting stdio MCP servers through client-side tool execution.

Since agents run on Letta Cloud, they can connect to http and sse MCP servers directly. This PR enables stdio server support by executing tools client-side via the CLI.

How it works

1. User adds stdio server: /mcp add --transport stdio <name> <command> [args...]
2. CLI spawns the MCP server as a local subprocess
3. Tools are registered with Letta Cloud (with require_approval=true)
4. When agent calls a tool, CLI intercepts the approval request
5. CLI executes tool locally via stdio and returns result to server

Changes

• Add src/mcp/ module for managing stdio server connections
• Update src/cli/commands/mcp.ts with stdio server registration
• Update src/agent/approval-execution.ts for client-side MCP tool execution
• Update src/headless.ts for MCP initialization and cleanup

Test plan

# Add stdio MCP server
/mcp add --transport stdio test npx @modelcontextprotocol/server-everything

# Test tools
echo "hello"
get environment variables
calculate sum of 10 and 25

Demo

All 9 tools from @modelcontextprotocol/server-everything tested successfully:

• echo, get_env, get_annotated_message, get_sum, get_tiny_image
• get_structured_content, get_resource_reference, get_resource_links, gzip_file_as_resource

Closes #150

[pmukeshreddy]

Testing Instructions

# Clone & setup
git clone https://github.com/pmukeshreddy/letta-code.git
cd letta-code
git checkout feat/stdio-mcp-support
bun install

# Run with your API key
export LETTA_API_KEY=your-key-here
bun run dev

Test stdio MCP

/mcp add --transport stdio test npx @modelcontextprotocol/server-everything

# Try these:
echo "Hello from MCP!"
What are my environment variables?
Calculate sum of 15 and 27

Should see tools execute locally via stdio subprocess.