feat(codemode): add secure-exec MCP executor

[NathanFlurry]

Summary

• add @secure-exec/mcp, a Node-hosted executor for Cloudflare Code Mode
• run each generated JavaScript execution in a fresh secure-exec VM with network denied by default
• bridge trusted providers/connectors through schema-validated secure-exec bindings and re-export the Code Mode MCP builders

Validation

• pnpm --dir packages/secure-exec-mcp test
• pnpm build
• pnpm run check
• pnpm --filter @secure-exec/mcp pack --pack-destination /tmp